Peter is correct of course.
Smalltime collusion may be hard to prove if it's done quietly.
But it's still not allowed.
I was at a small estate type auction that ran once a month. Small enough that the auctioneer announced before each auction that the help were all independent dealers who were allowed to bid. Any other way and he'd have no help. They all specialized, and rarely bid even within their own specialty.
Anyway, one night there's a bag of coins. And it gets bid up a bit, to maybe $40. One guy in the front row turns to another and says "Let me have this one and you can buy the next" The auctioneer stopped, gave them both a stern reprimand about how he could lose his license and if they wanted to do that stuff to discuss it ahead of time outside. Restarted the item at 45, got a bid from a new bidder and 50 from the other guy for the sale.
On the bigger end of things, here's an article about the settlement with a group of stamp dealers who ran a collusive bidding ring for almost 20 years.
http://www.ag.ny.gov/press-release/s...stamp-auctions
The sort of stuff they colluded on was similar, biggish to large lots where there would be limited competition from either collectors or from other dealers. Their ring made up a huge percentage of the buyers with both the interest and the funds to buy large lots regularly.
I have mixed feelings about small guys combining to buy a lot. It might sort of be collusion technically, but usually neither would be a serious bidder so the teaming up actually increases the competition a bit.
Steve B