I have bought a lot of AR-15 accessories on eBay. Not Chinese junk, I mean real Knights Armament M4 rails, Gripods, Aimpoints, Eotechs, Magpul stocks, etc. I cannot stand to pay full price, so I buy "as new" type stuff that maybe is open box but still has the box and such to verify authenticity.
Bought a lot of parts and accessories over the years. Over 789 transactions with 100% positive feedback, both buying and selling. Latest purchase Colt 1911 SS mags, 3 for $24.00 shipping included.
I've bought some AK stuff, some AR stuff (seemed like most of the bigger sites and the local shops were all backordered on some small stuff), some Glock stuff (extended floor plates and a Glock tool), surplus slings and some other surplus odds and ends to get a few more rifles to a more complete look. I'd say about 75% have been through people that have shops or sell through gun shows/out of their house. The rest were from hobbyists. Never had a problem from eBay or Paypal.