Close
Yahoo Japan + Cresent is generally what most people I know use for comiket stuff. Be prepared for some steep prices if it's something popular though.

http://auctions.yahoo.co.jp/

Deputies will do all the bidding and stuff for you (well, goody-japan certainly do, I haven't personally used cresentshop but I would assume they'd do so as well), up to the maximum amount you want to spend on an item, so you just need to worry about finding what you want on yahoo.