Can't these guys be reported for what's known as "fee avoidance"? Postal rip-offs have always been the bane of most Ebayers lives.
I always state my P&P costs loud and clear on my items. Only one person (our friend studioonemaster) has completely defied them, but he has been dealt with by having his bid cancelled, and banned from future auctions. Further action will follow, but I'm keping that under my hat for the moment (and I don't mean violence or extortion!).
Additionally, I am reluctant to bid on an auction that does not explicitly state P&P costs. If it's a really nice item, then I will mail the seller before the auction closes and ask them for a quote.