I'll add my 2 cents on this issue as a buyer with 2 disclosures:
1. This isn't directed at any specific seller, it is inherent in a PSA transaction on ebay regardless of buyer or seller "intention"
2. I don't know what eBay's mandatory return policy is or how it applies. I sell on ebay occasionally (mostly wackys), and don't want the hassle of dealing with returns, so all my listings have that caveat, and anyone that it doesn't work for is free to pass on my auction.
In purchasing PSA wackys on ebay, there are factors that make it difficult to judge the eye appeal of a wacky from most pictures. Most specifically, toning. There are cases where I have purchased a PSA wacky that looked great on ebay, but I had to flip it out because it was so ugly, mostly from toning, that I wouldn't want it in my collection. That can happen with tanning on the back too. PSA will also let some diamond cut cards pass through, some edge bends or kinks & other various things that can bother certain collectors even though PSA lets them slide.
The majority of PSA collectors only collect the grade, but there are some of us still out there that use the grade as the initial checkpoint to consider the card & then the eye appeal will dictate the thumbs up or thumbs down. I found the situation similar with PCGS & coin collecting, though I'm currently out of that hobby now.
With that said, I'm interpreting that as a top rated seller, you accept returns on other items, though that may not necessarily be true. Because of the PSA holder, it's easy to validate that you are getting back the card you sent, and no one can tamper with it, so there must be an issue with the item for someone to make a purchase, go through the trouble of paying and returning that they aren't happy with the sticker. I would guess that it's a "happy" purchase for most people that they want to make, not like changing a muffler or fixing a leaky pipe, so there is probably something wrong if you are getting a return.
Is it a common problem for you that people buy PSA graded cards & want to return them? What reason do they give when it happens?