There's aways a risk of buying a phone with an activation lock or black listed. Use the sellers feedback to vet out the ones that don't look kosher.
personally, I'd use swappa.com over ebay. I think its safer.
Anyways if you have a problem, you'll work through paypal not ebay. Document everything including any correspondences with the seller. The more info you provide paypal in a dispute the better your chances.
Are you kidding me? Do you know anything about ebay's money back guarantee? I bet not, because if you did, you would realize that if a buyer receives an item that was not in the described condition (or some details weren't mentioned in the description) then the buyer has every right to open a dispute through Paypal.
Really, just buy from someone with good feedback and you're safe.
Downsides of ebay: the dispute process can be lengthy, for example once you open a dispute the seller is given 10 days to respond, and from then Paypal will decide the outcome (which takes 2 days or so). Also, another major downside is the fact that it's not often that you end up getting a good deal - I don't know what it is, be it ebay's popularity which means there are many idiots or maybe its something else, but in most items you will end up:
1) not saving anything over retail price
2) saving can be so little, and on top of that there is shipping
So these other bidders often overvalue an item and the winner ends up getting a not so good deal.