Ebay "Private Auctions"

You can't tell that in a non-private listing.

You think you know, but you don't. It's almost impossible even for ebay to tell unless they're total idjits and use the same address or name.

When we were selling on ebay heavy you would have thought 2/3 of our listings had shill bidders....

I'd be wary of generalizing the experience you've had with one or two bad sellers as being representative of all sellers. If you really, truly believe a shill is in the mix, report the listing and look for another seller. With millions of listings, ebay depends on you and others like you to report bad listings.

There is no magical "shill bidding detector" algorithm that's reliable enough to be useful except in extremely obvious cases (same IP/different userid comes to mind) -- even humans can't reliably detect shills in live auctions.

All you (and the computer) can do is look for common patterns, and those patterns are not often noticed unless someone points it out by means of reporting; i.e., "seller constantly has same two bidders on all listings, and always makes second chance offer to me with the same item if one of those two bidders wins the auction.)

Keep in mind that you are collecting in a specialized market (Canadian large cent varieties) and it would be expected to encounter several others who share your interest in that specialty bidding against you. I have a couple of dealers in early American large cents whom I prefer to do business with when they list auctions. It would be entirely too easy for someone to notice that I am a frequent bidder on these sellers' listings, and come to the (wrong) conclusion that I am a shill bidder, when in reality, I'm merely a frequent buyer in a specialist market.