Recettear

- The best sweet spot for making customers richer, is at 104%, though some are terrible.

- Listen to the scrolling text. It gives you clues on when to place a lot of items on sale and get swarmed by customers.

- Sell red, buy blue.

- The numbers on items makes it easier to level up customers, so you have a use for slimes. Those numbers also influence how strong crafted items will become, up to 15.

- Never sell crystals. It is also smart to hold onto items if the store isn't selling it.

- Medicines give permanent buffs up to a certain amount.

- Customers start with pretty small wallets overall. As you make sales to them, especially ones near or directly at their preferred prices, they can level up (denoted by a little heart appearing). As they level, their wallets grow. Early on, little girls who come into your shop can be annoying because they can't afford some of the things they pick out, but sell fairly low (the 104% is a pretty good baseline) and this will ease up a bunch. (It can also help to keep one or two cheap items of each type in-stock just so that if the little girl comes in wanting something, you have one she can afford).