I've had a couple bad deals on Gunbroker, but so far not actually out any money, just had bad experiences. That said, the couple bad deals are totally buried in the hundreds of good deals. It's a price of doing business on the internet and getting bargains.
If the seller wanted a minimum price for their shotgun they should have used a reserve. It is entirely possible to just not get enough interest in an item and have to sell it for, maybe, three cents, without a reserve. But a deal is a deal and if a seller puts it in with no reserve then the deal is done.
Get your refund, leave negative feedback, move on.
One of my bad deals was a local seller with a Colt Cobra revolver at no reserve, auction closed with me the high bidder at a price about half what they tended to go for. Seller had lousy pictures and little information but since he was local I was willing to gamble.
When I tried to arrange payment and personal pickup (refused.) So I give my FFL info and request total with shipping, the seller came up with some lame story about how his gunsmith discovered the frame was cracked and by the time he had gone back to get it the gunsmith had destroyed the gun. So I told him that was OK, I still wanted the parts. So the seller said that the gunsmith had thrown away the parts. Then that same day, the seller put up another auction for the same gun, this time with a reserve. So I left negative feedback and moved on.
The seller left ME negative feedback and claimed I had never contacted or made an attempt to pay. LOL, I had all the emails.
Gunbroker support was not amused, and after they removed the bad feedback he left for me, they banned the seller.