Shill bidding is when someone bids on an item to artificially increase its price, desirability, or search standing.
Shill bidding can happen regardless of whether the bidder knows the seller. However, when someone bidding on an item knows the seller, they might have information about the seller's item that other shoppers aren't aware of. This could create an unfair advantage, or cause another bidder to pay more than they should. We want to maintain a fair marketplace for all our users, and as such, shill bidding is prohibited on eBay. For more details on what constitutes shill bidding, please see our full policy guidelines below.
https://www.ebay.com/help/policies/selling-policies/selling-practices-policy/shill-bidding-policy?id=4353