Outside of Greed Mode, the restocked item's cost is increased after each purchase.
For items, the price increase is 2 coins times the number of purchases. For example, an item starting at 15 cents would next be +2 to 17, then +4 to 21, +6 to 27, etc. The cost follows the formula 15+n(n+1) with n being the number of times restock has triggered.
For everything else, the price increase is 1 coin times the number of purchases. For example, a heart starting at 3 cents would be +1 to 4, +2 to 6, +3 to 9 etc. This follows the formula (original price)+(n(n+1))/2
The increase is not affected by discounts occurring naturally or through Steam Sale: costs will increase at the same rate, though the base price is still discounted. Once the item's cost goes over what it would naturally be, the price is no longer highlighted red.
Coins can be infinitely generated by buying Lil' Batteries. As Lil' Batteries only cost 5 coins, as long as Isaac starts with at least 11, the maximum of 99 coins can be quickly reached.
This cannot be repeated infinitely outside of Greed Mode, as Restock now increases the price of a shop slot each time something is purchased from it.
Deep Pockets allows this to once again be repeated infinitely, as the price will cap at 99 coins, while up to 999 coins can be obtained.
Steam Sale x2: Items can be infinitely bought from shops.
If a Lil' Battery is on sale, Isaac can buy a lot of items before the prices become too high in that shop.
Chaos: Restock is affected by Chaos, and can be used to obtain otherwise rare items.
Chaos + Steam Sale x2: Items can be bought infinitely, allowing Isaac to theoretically obtain every item in the game.
Keeper's Box: Buying spawned pickup or item in Shop will always restock into the same thing as what being sold in the upper left of the shop and their prices will never increase. This still works even if the top left slot already brought and allowed to change the top left spot with restock machine to items or your desired pickups. This does not work outside of the shop.
Member Card: Items bought in the separate shop will also be restocked.
A Pound of Flesh: Restocked items in a shop will still require hearts to be bought.
Shop, a new purchasable item is added to the store's inventory.
Drops 3 random pickups around Isaac.
Outside of Greed Mode, the restocked item's cost is increased after each purchase.
Steam Sale: costs will increase at the same rate, though the base price is still discounted. Once the item's cost goes over what it would naturally be, the price is no longer highlighted red.
Treasure Map, 
Blue Map, 
The Compass 
IV Bag/
Blood Donation Machine and an item which can spawn additional coins (
Counterfeit Penny, 
Swallowed Penny, 
Humbleing Bundle, etc.), infinite money can be produced by buying hearts for 3 coins and getting more than 3 coins back after donating health.
Black Markets, purchasable items generated via 
The Stairway and purchasable items generated by 
A Pound of Flesh that cost coins.
Purchasing an item that is on sale will replace it with another that is on sale.
2 of Diamonds + 
Blank Card:
Lil' Batteries. As Lil' Batteries only cost 5 coins, as long as Isaac starts with at least 11, the maximum of 99 coins can be quickly reached.
Deep Pockets allows this to once again be repeated infinitely, as the price will cap at 99 coins, while up to 999 coins can be obtained.
Chaos: Restock is affected by Chaos, and can be used to obtain otherwise rare items.
Keeper's Box: Buying spawned pickup or item in 
Member Card: Items bought in the separate shop will also be restocked.
The Lost, 
Tainted Lost, or if currently affected by 
Soul of the Lost / 
White Fire Place, items can be bought indefinitely.
Coupon/
Credit Card: Items made free will not be restocked, even while playing Greed Mode.
Mr. ME!: Items stolen by Mr. ME! will be not restocked, even while playing Greed Mode.
Concept drawn for Restock.
