^^^ you have our into words the feeling which keeps me from selling the boxed and perfect Turbo Grafx 16 that my neighbor gave to me, no strings attached. I haven't opened, played, or tested it (emulation of TG16 is sufficient for me).
The important factor with Moocher's Honor is time- for example, my SNES was a freebie from a friend, and the game collection included some notable titles- Zelda, Mario RPG, and especially Evo. If something came up & I needed to sell these, I would no longer feel guilty about it becuase they've been mine for over a decade, and I've lost contact with said friend. I have satisfactorily met the 'intent to keep & use' part of the code, and could no longer offer a return anyway. So depending on how long it's been & if your neighbor's still around, you may be in the clear!
Or maybe it's just laziness, because eBay selling is a hassle.
Ain't that the truth- it's why I still have that big box of anime I meant to sell a couple months ago sitting around.