I had one buy a new carburetor from me, then claim it was old and rusty (opened a claim against me without messaging me first). He demanded full refund plus shipping. I sold it free shipping, so I was already out the original shipping. I told him the terms of the sale were that buyer pays return shipping, and that the carburetor was new and I knew it was. He returned it and I refunded him. It was fine. Not one spot of rust. It did, however, smell like gas (he used it). I sent him a message asking him what was really wrong with the carburetor so that I could figure out if I was going to relist it or not. He replied, saying it wouldn't adjust right. In other words, he has other problems with his equipment, not carburetor problems and he wanted to force me to refund his money. So now I have a carburetor that is not new anymore, and I'm out the shipping.
I had another guy I sold a carb to and when he got it, I received a message saying, "The carburetor came today and there was a tear in the package it was in. Now it's not going to work because there's no telling how much dirt is in it and I want to know what you're going to do about it". Lol. I wound up not having to refund his money because it worked after all.
Then you have the ones who bid up your item and never pay and tie your item up for however long it takes for the unpaid item case to be paid (you should lose your ability to buy on ebay for a month after not paying for an item, maybe longer).
There's also the guy who clicks buy it now on your item, then sends you a message saying nevermind, he found a cheaper one somewhere else.