I sold some of my stash because it didn't fit - trusted the stated size range and got diapers too small for me. Sold many bags to people skinnier than me, for much below retail, rather than throwing them away. Win-win. I don't have to take such a loss, and they get a good deal. Don't feel bad for the seller, odds are good you did them a favor.
What I feel worse about is hearing people finding good ABDL diapers in thrift stores. There's a good chance theres a story behind that - parent finding their kid's stash and getting rid of it, spouse saying diapers go or I go, purge slope of the binge/purge cycle, etc. Still a chance of someone just not wanting to try to sell locally and donating them instead I suppose, but I'm always expecting it to be something less fortunate for the former owner.