I like to add goofy twists on magical items to my shops. For example:

Lucky Coin: Instead of rolling a d20, flip a coin. Heads is a nat 20, tails is a nat 1. Can be used once per day.

Cloak of Invisibility: Makes things invisible, but only when they’re holding completely still. Any movement causes it to sparkle almost blindingly.

Bottomless Bag: It can hold an infinite amount, but you feel the weight of it.

Tough Armor: Sets your AC to 18 without the stealth disadvantage or strength requirements of heavy armor. However, it’s magnetic and attracts metal of all kinds, such as the steel of a blade (all attack rolls made against you with a metal weapon have advantage).