Yeah you must watch many times a seller will himself or have a buddy up the bid to make profit. Now yes this means that if no one else bids then the seller must pay Ebay thier fair share of the money but usually at no real lose to the seller.
Watch the sellers other auctions and see if you notice the same buyer on a frequent basis "upping the bid" this is the first clue.
Another thing most seller that purposly up the bid will often times send you a "second chance" email, asking if you want to buy the item for your highest bid amount. Usually with the excuse that the buyer was unable to keep up with thier end of the deal.
Not all sellers are "crooked" like this but many are you just need to watch for the patterns, and if your positive this is what is going on report it to Ebay, it is against thier policy to up bids this way

I have fell victim to this many times and got the "second chance" email and declined it, just to see the same item up for bid again with the same person who failed to meet the purchase agreement before placing the high bid.
Buyer Beware and dont let un-ethical sellers ruin the great buys you can get on Ebay!