No one HAS to complete any transaction
Unfortunately, even if you are high bidder, the seller doesn't have to complete the transaction. They can simply not respond to your emails or refund your PayPal payment and say that the item was damaged and not longer available. The worst they can expect is your leaving them negative feedback for the transaction. Of course, expect them to do the same to you. If they have a really bad track record and you can prove that they're scamming you, they may be barred from selling on eBay. Chances are, they'll just open another account, under a differnt seller name and be off and running again.
My experience is that most sellers are honest and don't want to waste your time, or theirs. I've been buying on eBay since 1999 and with over 1000 purchases have had only one situation that required my using eBays mediation services. It worked and the transaction was completed to both our satisfaction.
Hope this helps.
Dr. C