I have been looking for Coopers for several days on Gunbroker.com, Gunsamerica.com and Gunsinternational.com. I have bought from all three, but only Gunbroker.com is an auction site. I have seen bidding on items that started low. On items starting at closer to their actual worth, you will often see a flurry of bidding right at the very end. People don't want to tip their hand, or they're hoping nobody noticed the item they want (yeah right).
I have sold on Gunbroker.com but I used a dealer who sells on that site a lot. He does great photos and descriptions, and he has tons of good feedback. It was well worth the 20 percent commission. It was amazing how much the items sold for.
I just bought a Cooper tonight on that site. It was a "buy-it-now" deal. He was also open to offers, but I thought the asking price was fair, so I took it.