What’s the amount they take cards for? Is it market price, under market price or above market price? (This is for yu gi oh not MTG but MTG answers would be helpful I play both)

Depends, cards below a certain value will be classified as bulk, high value cards they will research in real time and make individual offers, mood range cards will have a more standard value at about 50% market.

I'm answering for MTG here, but if you mean market price as what you can buy a card from TCG Player or the like for I can usually count on that I'll get half of that when I sell to the store. You get a bonus if you sell for store credit. You'll get a higher percentage from cards that will move fast while they'll likely pay a smaller percentage for cards they'll have to sit on a while since their money will be tied up until they sell it.