My pet peeve is when a seller wants an outrageous amount for a beat up machine that doesn't work, has been stored (usually improperly) for years, or even has a broken backglass or the playfield is badly worn down to the wood.
"I've seen these go on ebay for $$$$$" they say.
At that point I just say "Thank you, but they're not for me. Good luck with the sale." and walk away.