“Marks on the underside”

If you disclosed it and said "slight discount for minor scratches on the bottom". I think that would have covered it.
Along with no returns unless DOA. Put on tamper proof tape... I’ve dealt with few sleeeeez balls..

If it’s packed right, and tested before shipping.. VERY few DOAs, Like NONE... in 45 years... I did have a Schitt Freya +,  bad out of the box though... Super heated... WEIRD... It just wasn’t checked with the valves they sent... BIG bo bo. Always check. Even the cheapo PC, YUP, check it, or expect it.... What I use to tell the apprentices. LOL

Regards

Yes any marks are very bad.

Unfortunately, losers pop up on Ebay.
I sold a vintage camera lens and the buyer reported it not working.
He ended getting a working lens and I got his garbage lens in return.
I knew he was scamming me as he said it happens every time.
I reported it to Ebay.
In any case, letting him return the unit was the best thing to do. I would try reselling it on USAM or Audiogon.
Bob

When selling equipment make the best description but show any blemish, ebay has many kind of buyers.
Buyers there show less sensitiviy on cables, tubes, music but more on gear.
G

Never had a prob on eBay buying or selling.
oldmecs advice right on.
NO RETURNS EXCEPT DOA weeds out the users that just want rent free.