I'm sure they could work out strong probabilities based on bidding history and IP addresses.
Not really. Both are pretty unreliable. Here is a screenshot (with identifying info removed) of members with duplicate IPs here. This is only about 1% of the complete list of the list.
Bidding history is just as unreliable. We used to sell on ebay a LOT. There are 2 reasons we stopped. 1. Listing fees. 2. Buttheads with no clue saying we're shill bidding.
We used to go to shows all the time and handout ebay business cards. Handed out thousands of them. All the time we heard, "We don't do ebay." We'd say well you always stop at our booth so just look at our stuff. So we ended up with dozens upon dozens of people whose only purchases were from us. Can't tell you how many time we saw know-it-alls on forums just like this swearing they had PROOF we were shill bidding.