I am a private party and I buy and sell on both sites with unblemished feedback rating on both (312 on ebay to date).
I just got a PRI Gas Buster M84 charging handle off of ebay for $71 shipped. As this retails for $89 + shipping on up, I got a decent deal there. You have to be careful, though.
Alot of people like to stretch the truth on their parts. I have a box full of parts which includes about 4-5 receivers, 5 barrels and a bunch of misc parts sold to me "AS NEW" or excellent condition only to find out they are pitted/painted or not milspec so the finish is poor or some backyard, hack gunsmith has FUBARed the thing beyond any cost effective repair.
I just took return on an upper I built for a police officer in Texas. I built it with a "NEW, MILSPEC" upper receiver, or so I thought. The finish was not milspec and was coming off in large pieces inside the receiver while he was firing the upper. This receiver was not milspec anodized, rather it was powder coated or some less resistant coating process. Now I am out a customer, and I am out the money I paid for the junk receiver. As most of the repuatable companies want about $100 to anodize a receiver, I would now have about twice what it is worth into this receiver just to get it back to spec. So it sits in a box until I figure out what to do with it. Bottom line, I am a seller who backs my items. I am going to eat about $100-150 on this build due to the receiver, but that is business. I jumped on a good deal on the receiver only to find out it was a POS. So I took responsibility for it.
There are honest and reputable sellers out there on both sites. You will always hear about the bad ones, but rarely hear about the good ones. Noboby bitches too loud about a perfect, smooth transaction.