I went to a local new/used retro game store. I brought in a GameBoy Pocket. I gave one of the employees the GameBoy and he said, "There's a line on the screen. I'll sell it for $5 and give you $5". He gives me the $5, I go on my way. I came back two hours later, same guy is working there, and I go see if my GameBoy is still there. It is, and it's being sold for SIXTY FIVE DOLLARS AND 99 CENTS! I tried to complain, but the guy said, "Oh, you didn't trade that in. I never saw you come in today at all!" Did he scam me?

Not a scam - but very bad business practice.

you agreed that there was a line on the screen and you agreed to $5. Does not look like you have a case, although you got way too low a price. happens every day. beware.

No. You got what you agreed to.

He took advantage of you not knowing what it was worth.

You got ripped off. Don't know if that's illegal though.