Since the OP asked about selling guns...
I've sold a lot of guns on Gunbroker. The only problems I ever encountered were from buyers in Cali. bidding and winning guns that are not on the Cali. DOJ approved list. Once a gun sells you are charged a sellers fee based on the actual selling price of the gun. If the buyer cannot or will not follow thru you are still stuck with paying the sellers fee. My adds clearly state no sales to Cali.
Shipping cost can be very expensive depending on the value of the gun. I am not an FFL so I only ship thru an FFL to the buyers FFL. I'm charged the cost of the medium flat rate box ( ~$14) plus insurance (~ $1.50 per $100) plus $25 transfer fee. So, a $1000 gun cost me about $54 to ship thru my FFL. The same gun thru UPS or Fedex overnight would be about $100 depending how far it ships to. I usually charge about $30 to $35 and eat the rest.
I only accept U.S. postal money orders or certified bank checks. There is a phone number you can call to verify the money order and you can call the issuing bank to verify the bank check as legit. The buyer is responsible to provide you with a copy of the FFL's licence or having their FFL email a copy to you.
Take lots of pictures. People want to see what they are buying. If you were buying a gun on line what pictures would you want to see? Give a good, accurate description.
One last thing, Gunbroker and the other auction sites are only a medium for listing your adds/auctions. You are pretty much on your own if things go wrong.
Online feedback can be generated ...it may not be real
The only way someone can leave feedback on gunbroker is to win the auction. Considering how much I have to pay in sellers fees everytime I sell a gun it would cost a fortune to build up fake feedback. OTOH, if someone sells a bunch of cheap stuff to build feedback and then offers an expensive gun for sale I would be leary.
Jim