I've sold a few hundred guns on gunbroker.com in the past few months.
After an auction closes, if I have more of the same thing I want to list again, it is a lot easier to just "re-list" the item than to upload all the pictures and enter/format all the text and do a "new" auction. It doesn't mean that I am trying to sell anything out from under the auction winners, it just means I will spend 3 seconds putting up an auction instead of 5 minutes. Do that for 10+ auctions and the time starts adding up.
Also, as others have said, all the contact information is provided to both the seller and the buyer when the auction closes. I usually send an e-mail detailing my office hours and further payment details, but I don't know if it is necessary. I've had auctions end and the phone ring with someone wanting to pay by credit card. I have also had auction end and the people want to have their buddy come and pick up the gun. I even had one guy (this was after giving him my office hours several times) supposedly come by in the middle of the night at I wasn't here and so I could just ship them to his house if I wanted to get paid for them.
Overall, 99% of the buyers have been great to deal with. The most frequent problem I have is people not understanding how dutch auctions work.