you mean Shield bidding. This can be proved by ip addresses and involving Ebay in the investigation.

There are also a number of buyers and sellers that do this. I remembered selling an item and some joker just bidded very high and did not communicate later. It pisses me off as I found out the winner did not even touched the account for 1 year so yes, there are sellers who use this method to crush other seller's items so that their becomes the next best.

I suggest you report to Ebay if you feel that there is an unusual pattern.