This mod aims to carefully expand the range of items available at stores, while also liming the amount of items available.

Main rules are:

1. Vendors sell a wider range of items, sensible to their profession, and target other characters besides the player.
2. Items vary by season.
3. Items are not as predictable, many of them are available randomly.
4. The amount of items available is restricted.

Every shop has a settings that can be set to "Standard", "Hardmode", or "off".

"Hardmode" comes with lower stock amount, sometimes higher prices and lower chances of rare items. Changes to every shop can be disabled individually. Compatible with Generic Mod Config Menu.

Saloon

Gus is trying to cut costs to make ends meet, so he primarily cooks from seasonal ingredients to save up.

Prepare for Parsnip Soup in the spring, Pink Cake in the summer, Pumpkin Pie in the fall, and Trout Soup in the winter (just some examples, actual variety is bigger, random rotation).
There are also some dishes available year round too, such as Pancakes.
Additionally, he serves alcohol, caffeinated drinks (and occasional Ginger Ale too).
He is but one man, so the amount of food he is able to cook is not limitless. He serves more food on the weekend, starting with Friday. And his stock can increase after certain events.


Pierre aims his store to be the one-stop shop of the town (but maybe falls a little short).

He sells seasonal seeds (expanded amount, including Strawberry Seeds starting from Year 2, as well as the new 1.6 added seeds).
But the seeds rotate on a random basis and the stock is limited. Meaning you may arrive to the shop in the spring to only find 20 Parsnip Seeds, and will have to come back tomorrow to have a chance to buy some Kale Seeds instead.

His stock starts as more limited, but can increase after a certain event.

Those without a green thumb come here to buy Bread, sauces, Flour, Sugar and such (also limited amount in stock), or seasonal vegetables and flowers (much less of the latter).

Caroline also helps out the family business now, by offering some Tea Leaves for sale from time to time.

Festivals

Items from Pierre's stands are far less predictable and rotate randomly (except for the Stardew Fair, the one with tokens, it is left as it was).

Notable changes: instead of Strawberry Seeds Pierre now sells actual Strawberry. (Seeds are sometimes available at the shop instead.)
There is also Green Tea available as a refreshment.

Adventurer's Guild

Marlon has positively decided that adventurers need help taking care of themselves, so he now sells Life Elixir and Garlic Oil (limited amount per day).

He also sometimes offers a random Mineral and Gem (for a completely unreasonable rip-off price, but don't tell him that). So if you are looking for a gift, or need help with the Museum's collection, he can help you out (he does not sell anything impossibly rare).

Additionally, he has a bit of monster loot to spare. Another notable change: a certain special weapon can no longer be obtain for money, and instead requires a rare item.

Marnie's Ranch

Marnie now does not have an impossibly large amount of hay to spare. She has some, but maybe it's time for a Silo.

She offers a moderate amount of various Milk, Eggs, and Cheese, with occasional extras.

More complex items for sale are now more rare and on random rotation, she is running a ranch after all. Swing by her house more often if you desire a Dog House or require a Heater, don't count on it being in stock every day.

Blacksmith

Clint would rather sell you Metal Bars and not ores. He is smelting them for a living after all. He has Copper, Iron, and Gold Bars in stock (a little ore on rotation too, but he doesn't like to share).

Additionally, he now sells Lightning Rods, Batteries, Starting and Quality Sprinklers (only those two levels, no higher), Iron Fences, and... Bombs.
Marlon still doesn't trust him to make weapons for some reason...

Some of Clint's items for sale depend on your progress in the mines, or even weather.

Carpenter's Shop

Decorating the house has never been so immersively inconvenient. Furniture has been divided into categories and randomized, what shall Robin have in stock tomorrow - nobody knows. The amount of items is also limited, as she can can only make so many items.
She also sells big craftables (machines).
As for the materials, Robin can not be bothered to spare the materials she needs on her craft, so she only sells a very limited amount.

Her stock varies depending on the day of the week! See spoiler, if you want to know.

Casino

This part of the mod is an outlier, has nothing to do with limiting the stock, and therefore is disabled by default.
Only adds Treasure Chest as a reward for 500 QI coins, allowing you to exhange your winnings for money (by selling said chest for 5k Gold).

It is basically an updated version of Treasure Chest at Casino by Schneitizel. If you want the higher difficulty of the original mod, set "Casino" to "hardmode" in config.

Dwarf

No longer sells bombs, but is willing to share some juicy Bug Steaks instead. Also has a little of random small loot to share, including monster loot.
Offers either one or the other decorative item he normally has, randomly. Still offers Elixirs, but in much restricted quantity.

Fish Shop

Willy sells a reasonable amount (!) of random common (!) Fishes of the season. He also sells some Algae of random variety, and a small amount of bait.
Fishing related stock is limited in number and randomized (which bobber will he have in stock - nobody knows).

Hospital and Ice Cream Stand

Nothing unusual here, except for the limited amount of remedies available per day.

Ginger island Resort Bar

Is essentially a much smaller version of the Saloon inventory. Gus still cooks seasonal food, but he doesn't bring that much with him.
No Mango Wine, varied drinks instead.

Sandy's Oasis

Sandy sells seasonal seeds for a pretty penny (how many farmers you reckon come by the place?). That includes possible Coffee Beans, too.
For the common traveler she offers a small stock of snacks instead.
Her furniture items rotate randomly.
If you are on the lookout for a seasonal plant, she has them all (but you'll never know which one).

Krobus and his Shadow Shop

More item amount limitations, some items are randomized. Heftier price on a creepy new item from 1.6.

Joja

They have the largest stock. The seeds available are still randomized and seasonal.
They have vegetables, both seasonal and off-season (the latter being pricier, of course).
They have Fish (a little), Eggs (not large ones), Milk (only Cow Milk). As well as other groceries too, as usual (no Tea Leaves, however).
How could you possibly resist the temptation to shop there?..

Compatibility

Unfortunately, since this is a very manual edit, mods adding new items to shops would need to be patched to fit in (unless the author/s already made their items limited in amount, which is so very fantastic and compatible by default).

For more details on the patches see their changelog. All patches can be used independently from the main RSSI file. Every patch has its own version history, which has nothing to do with the main file version.

Some mods that are compatible as is:

East Scarp
Ferngill Simple Economy

Patches available on this page for:

Adventurer's Guild Expanded (1.6)
Animal Husbandry Mod (former ButcherMod) (adds Meat to Joja and - this can be disabled in config - other stores too)
Bird overhaul - Ornithologist's Guild (Joja has less Binoculars)
Cape Stardew (additions to Pierre's only)
Cornucopia - Artisan Machines (adds new items to stores)
Cornucopia - More Crops (full version only)
Cornucopia - More Flowers (full version only)
Downtown Zuzu
Drink Water and Eat Frozen - Kettle and Microwave (CP and AT) 
The Fishmonger - Custom NPC 
The Love of Cooking (1.6)
Lumisteria Visit Mount Vapius (groceries)
Ridgeside Village (Kimpoi and Heaps only)
Stardew Valley Expanded (vanilla shops only)
Strange Machines Revisited (materials only, otherwise simply use mod's own config)
Pet's Enhanced - A Pet Mechanic Overhaul

More patches are also available here: MRSSI - More Realistic Seasonal Shop Inventory by JohnMarley2018
and EMRSSI -- Even More Realistic Seasonal Shop Inventory by Stunky McDoo


Incompatible:

Price Buffs (and similar): any mod changing prices of items will conflict.
Robin Sells Machines, Battery Pack From Joja (and similar): RSSI already includes this functionality.
Stardew Valley Balance Overhaul and similar: any mod changing shops will add and rearrange items, making the stock unlimited again.

My other mods.