eBay can’t verify that either buyer or seller is telling the truth.

They automatically assume both are honest (even though that can’t be true) until they have built evidence to the contrary.

So…

If you sell a lot of items and get a lot of returns for not as described, then eBay will use that as evidence that you are trying to defraud customers, and will almost certainly ban you from selling.

If a buyer returns lots of items from different sellers for not as described, then eBay will use that as evidence that they are trying to abuse the system, and will probably ban them - though I think it will probably take more evidence than a seller would need to be banned. That is why, as a seller, reporting a buyer who has clearly abused the return process is important, as it would have more weight than pure number of returns.

Edit:

But note this comment is about most sales on eBay. Personally, I do not understand why / how they can take this approach when the Authenticity Guarantee is used.