Not much since the price of them dropped from $599 to $499. From what I see, an HTC One in good shape, maybe with a few extra accessories, can get you $400 or a little more or a little less, depending on if you have the original box/any scratches/phone and accessories in their packaging. Surprisingly I've seen HTC One Maxxs sell for $450 for ones that are in near perfect shape. With the taxes and fees that ebay and paypal add to the sellers, if you sold an HTC One for $400, you would really only see about $350 of that. I think it's a better deal if you can sell one locally, to either a friend or family member, possibly through friends on Facebook. Something that I am considering doing to my HTC One, so that I can upgrade to the HTC Maxx.
I have an HTC One that was S offed by Sonic with a java card. I have not seen any posted. I imagine that if I labeled it as " Being able to work on almost any carrier." I could probably fetch up to $450 or more for mine. With the right knowledge and understanding on this phone with it being S off / Unlocked / Rooted it really could work on any carrier. straight talk, AT&T, page plus, Viaero, Tmobile, and sprint, since I believe that it is also a worldwide phone that has all of the GSM and CDMA bands inside of it.