It's not easy to offer advice as it depends on things that aren't clear from your post. First of all, was the seller a private or a business seller? If they were a private seller what was the returns policy on the listing? Then what return reason did you use when you started the process?
Ultimately, now that you have closed the return, you can't reopen it and ebay won't be interested in the slightest so any further action you take would have to be through your payment provider. But it may be that you don't have any right of return anyway in which case the best thing to do is to sell it on yourself.
If the seller was a genuine private seller and had no returns on the listing then you don't have a valid reason for return. As the item isn't faulty but just incompatible with your set-up (unless the seller's description specifically said that it was compatible with a MAC) then that would be classed as a change of mind return which is only compulsory for business sellers. If the seller's description did say that it was MAC compatible, only then would you have a valid not as described claim.
So, assuming there was no compatibility statement on the listing then you would only be able to return if the seller was a business (or a private seller who had change of mind returns accepted). You should have then selected one of the change of mind reasons rather than one of the seller at fault reasons (like item doesn't match the description or is damaged/faulty etc). You would then be responsible for covering the return postage yourself.
If you wrongly selected one of the seller at fault, item not as described reasons then ebay would wrongly make the seller pay for the return postage. If that's what happened then that may be why the seller got nasty as there is no process for sellers to challenge false not as described claims at this point. Sellers can report buyers for false not as described claims but ebay don't actually do anything.
So, as you can see, there's lots of possible scenarios which affect what you should/could do and who's right here. Buyers are very well protected on ebay to the point that it's virtually impossible for them to ever lose a return case as long as it's within the return timeframe. If you hadn't voluntarily closed the case then you would have won, even if you were in the wrong (which is impossible to tell without seeing the listing).
