This is my story, too... early follower getting great deals, and cheap shipping, but later swamped by the crowds, so I don't buy much from him anymore. My problem is his success, though, so good on him...
The other tactic of Peter's I would note is he offers small groups of similar titles in each auction. He doesn't list a single copy of Superman for sale, and he doesn't list 400 issues of Superman for sale... it's consistently 25-30 books with great combined shipping offers, so he never cannibalizes his own market.