I'm curious if you have any follow up on this Malcolm? Some time ago, a relatively newer ebayer had emailed me concerned her account was compromised (she had won an item from me, which she had bid on herself) I quickly searched her ID, and sure enough, it had been hijacked and was being used to sell satallite systems, etc. I pointed her to the auctions, and she promptly contacted ebay. Not sure what happened in the end, but she did pay me, as she had been the one to bid, not someone else, so a little different situation.
Your bidder had 5 bids on out-of character items, involving 4 different sellers. Definitely really odd, but nothing being sold under the ID, so not what you would expect from a typical hijack,