I agree on the suspicion on T5 being a shill bidder or consignee. As you mentioned the first flurry of bids are his alone. If that were a real bidder why wouldn't he just stay at the first bid until someone else is bidding against him.

As for the buyers premiuim. I have now seen auctions as high as 27%. It hurts but if you need to fill a hole in your collection where else can you go?