They will buy for store credit that you can use to get more cards. You can ask "how many packs can I get for Purphoros?" It depends on the person though. If you're trying to barter MTG with the Yu-Gi-Oh guy for example, he's less able to be flexible.
They buy and sell Magic the Gathering cards, typically at the TCGPlayer buylist price for cash and 25%ish more if getting store credit, from my experience (I cant verify this). Their interest in purchasing cards will depend on the card(s) specifically
They buy, but don't barter. I think they normally buy at 80% of tcg low. Doesn't hurt to go in and ask though, they're all nice people
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