Some sellers have repeat buyers. It's not unusual (although it is rare) for a new buyer to have one seller he sticks with for several months because the seller is giving good service.
In some categories, there are only a few buyers and sellers. Those will naturally be dealing with each other a lot.
But if what you are talking about is 'shill bidding' in which a buyer drives up prices for a seller (they may even be the same person) most reports of this come from a misunderstanding of how eBay's automatic bidding works.
The way to tell is there is a shill bidder is to realize that the shill never wants to win.
The shill wants you to win, but at your highest price.
So look for bidders who consistently retract bids near the end of an auction, leaving the underbidder as the winner.
If someone is consistently beating your bids, raise them.
However, the best way to avoid the whole question is to ignore auctions altogether.
Buy at Fixed Price.
You will know what you are paying before you order (including shipping costs) and you will get your item faster since you don't have to wait until the auction ends to see if your offer will be successful.
