Sharipe w*****. This drives me mental. Put a sticker or a tag on it, DO NOT write directly on anything with sharpie, it is either a pain to get off, or its never coming off.
Over pricing junk.
Asking for huge prices for toys, especially if it's a bag of crap with one G3 in it.
Putting a sticker over a flaw / damage. (I have been a victim to this more than once, I now pull stickers back to check.for damage, I don't care if they think I'm trying to swap them, I think I have the right to know the it ems condition)
Hiding damaged toys in a bag of junk (where they show the decent side of the item but the other side is trashed)
Selling items that don't work / haven't been tested.