Taking photo, best in a shadow place outside, no direct sunlight, and not dark. Using a light grey cloth as background being the best. Turn on 'micro' function on the camera, turn off flash, focus on the item, hand not shaking, control breath,,, not very different from shooting a gun
advertisement... write a text description on the item, post pictures of it, along with asking price, shipping and returning condition. Some viewer may ask questions, answer those questions, and post more pictures if requested.
sell... if you're not licensed, better do the shipping step via a licensed dealer. Ask buyer to send his FFL to your transferring dealer. Pay check could go to you or dealer. Dealer charges you a fee on this and dealer will ship the gun to buyer's FFL address. One tricky part of this: (1) don't send the gun out before pay check is cleared (unless you know the buyer well); (2) if buyer decides to cancel buying, don't refund buyer until his pay check is cleared (again, unless you know the buyer well) -- in rare situation, there was low-life person cheating money this way, be aware.
return... no experience. But upon receiving the returned item, check it carefully, make sure it's the same item in the same state that you sent out. Then, write a refund check to buyer.
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For buying,,,, once I bought a nice Red 9 on auctionarms.com, cheap. The seller behaved a little bit weird -- after auction, he pushed very hard to pay him fast, "best via wire transfer"... but he only cares money, never asks for FFL... that alerts me, checking his feedback, positive, a few A+++ on $2, $3 small items in one month. Very suspicious. I reported to auctionarms, and they replied "Don't pay, the pictures posted were copied from an auction house catalog", and banned the seller.