I don't think this is a shill bid, I see this all the time on GB and eBay when someone places a bid and then thinks he's going to be outbid and keeps increasing their bid just in case someone else bids not realizing you can just place one high bid and be done with it? They don't understand the bidding system.???
Enough of these absurdly high priced SP5 CDNN sales went through and have positive feedback for the buyer that I don't think there's anything to the conspiracy theories here. You're talking about CDNN paying a few thousand to shill bid... Not sure the math really works in their favor. It's also pretty hard to register extra Gunbroker accounts compared to many other online services, they usually want you to verify you are in fact who you say you are.
There are a lot of people who have no clue how to value things, and some of them inevitably do the research after the fact and welch.