Unfortunately there are people out there that will abuse the system. I think the biggest thing is to not let them ruin any of this for you. There are a lot more decent people out there, so don't let the stinkers win! One of the best pieces of advice I've heard about returns is to offer free, no questions asked returns. If someone messages you about needing to return something, you can accept and they return it. A lot of the time they won't even go through the hassle. Best case scenario, you get your item back to resell, worst case scenario they send something back that isn't yours. In this case, reach out to ebay, find a way to reach them by phone! They are there to help in those instances.