We rarely know the real truth of what is going on with a return, but let me ask a question. How would you want to be handled? What would you think is fair if you are on the other side of this exchange?
I happen to be married to a better half who rarely chooses to keep the gift I get. Our daughter and I have come to get a laugh out of it each holiday season and don't let it get the best of us. It is the thought after all, but things might get more serious if the merchant would not let us return it. In fact, if I have doubts, I confirm before I buy, but this could be new for them.
As others have mentioned, if the seller wants to get nasty about it, there is a good chance eBay and/or PayPal will side with them. (Regrettable, but seems to be happening more and more)
There is certainly the possibility of a scammer sending the wrong product back but there are methods to discourage this, and a restocking charge might go toward discouraging the return as well. You could explain that the restocking charge helps to defray the extra handling and videotaping of the package being opened and item inspected.
Done in a friendly fashion, we have rarely had an issue, but there is always a first time, so you have to go with your gut. If their feedbacks seem to say they are honest, I would take a shot. But just MY opinion.
If they have been on for 5 years and only have a few feedbacks, they are either not buying much or sellers are not leaving feedback, which might be a red flag. There are sites to check for feedback OFF eBay, and maybe someone here will mention it. I rarely use it and it has slipped my mind.