and the last 125 feedback received as a seller have all been from the same disguised buyer
Well, if he thinks he is 'shill bidding' his items, he's doing it wrong.
In shill bidding, the seller does NOT want to win. If he does, he not only does not get paid, but he pays out fees to Paypal.
Sounds more like he has an enthusiastic and loyal customer.
The 'disguised buyer' is not a problem as such. The only ones who can see a bidder's real ID are the bidder herself and the seller. If you bid on an item, your ID is disguised too.