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SCAs with any form of collusion (price fixing, bid fixing, etc.) - rarely is it clear cut. If I'm at an auction, especially a live one, I might employ tactics to get what I want. I can't tell the guy next to me "I won't bid on this one if you don't bid on the next", but I might decide not to bid on the first item, figuring he'll buy it and then there will be one less bidder for subsequent lots.
If it's someone I know at the auction bidding, I might decline to bid out of courtesy - it can be rude (and costly) to run up someone you know, who may think you're doing it out of spite. Equally, I'd expect the same. It's a fine line, and probably not illegal (at least could not be found guilty in a court of law) if there is no quid pro quo conversation. No different than seeing a buddy with a high bid on ebay (pre-user ID changes) and deciding not to bid out of courtesy, and hoping the same is returned).
One of the most obvious cases of collusion in this industry I've ever witnessed firsthand was when I was running vintage email auctions about 10 years ago. A group of collectors, many who bid in my auctions, decided to compile a list of their bidder numbers and distribute it amongst themselves, so they could avoid bidding each other up (I listed the high bidder number next to the current high bid in the listings.) I found this out when a friend of mine who was in the group forwarded the list to me.
The real kicker was when one of the collectors emailed me, asking if I could change the group's various bidder numbers into a single sequence that would be more identifiable to them at a glance. I've never seen such boldness before or since!