Klaus,
There are several hallmarks of a shill bidding seller that were easy to recognize, back before eBay hid bidder identities once an auction had closed. I have had success in taking down auctions and even sellers with well-researched alerts to eBay.
The first hint is the same buyer bidding on multiple unrelated items from the same seller. That is enough to arose suspicion, but usually not enough for eBay to take action. Some sellers do have legitimate customers who buy a lot of items from them.
The next behavior to watch for is loser bids. Most experienced bidders know to add a few pennies to a bid, to increase the chance of beating an inexperienced buyer who bids in even increments. Loser bidders do the opposite, they will put in bids at $99.99, for example. Often you might see a sequence of increasing bids ("nibbles"), each a "loser bid". The shilling seller is trying to extract the highest possible value for the item by forcing the legitimate bidders to pay their maximum bid.
If the shilling seller goes too high, then you will often see a bid retraction, just for the final bid. That will leave the penultimate shill bid just below the victim's high bid.
Finally, you can review the seller's feedback. If they have received positive feedback from a buyer immediately after a sale of goods, that is a obvious indication that it was not a legitimate transaction, since shipment and receipt cannot have occurred in 10 minutes! Avoid such sellers at all costs.
Be careful out there!
Regards,
Jon