I agree on both accounts! I will gladly build all the sets I still have on ebay and then sell them as new!
Also some of the sets that are sold as 'new' without boxes means they were taken on of the trash. Was it someone here who said LEGO Store returns would be thrown in the trash? So some of them are real LEGO and people are dumpster diving. Which I'm more than willing to do, just make sure that the dumpster isn't one that smashes things.
That was me - at my store it was open returns are to be thrown out, full stop. YMMV, but if other managers are as fanatical as mine was, then returns aren't getting tossed still sealed in the individual numbered bags - bags got cut open and loose pieces then dumped straight into the trash. I doubt anybody is dumpster diving whole sets out of that. I'm sure some stores are less rigid about it, or maybe there's even some outright fraud going on where sealed bags are getting tossed for later pickup, but I can't imagine either avenue is a significant portion of the resale market.