“Marks on the underside”

The only thread so far that I agree with all the posts above.

Bad luck selling to that buyer, but you saved yourself some hassle. Not worth fighting it.

That's insane...and I'm one of the OCD ones.  Almost every single piece of new gear I've bought has had a minor blemish somewhere.  In fact, I just received a Denon DCD-110A today and it's fit/finish is exceptional...yet even it has a minor scuff on the bottom.

In addition to the Denon, I can distinctly remember minor blemishes on the bottom of a brand-new Panasonic DP-UB9000 and a brand-new Matrix Audio Element X, both of which I recently received.

It's not reasonable to expect the vast, vast majority of even brand-new devices to be defect-free on the bottom.  That's the part that's, ya know, unfinished!

Your seller was just trying to back out of the purchase.  What a douche move.

I sell on eBay. An accurate description is critical and that includes marks anywhere. If I were selling an items with blemishes on the bottom which you normally can’t see I guarantee I would point that out and show it in photos. 

Surprised he returned it (he had to pay for that, right?).   I've had buyers (and I'm sure others here have as well) who will agree to a price, then will use the "there-are-scratches" routine to get you to lower price at last minute.  All you have to do is say "No thanks.  I'm not worried about selling it."  But if I read an ad like that and wanted to buy it in good faith, I would DEFINITELY ask first for a description or picture of those blemishes.