A few years ago, I bid online for a poster at Profiles, and the whole thing ballsed up. Nothing went through.
I am sure since then things have been improved. I believe they were just very stringent on which browsers worked and didn't work. I was on the latest version of IE at the time... so, who knows why that wouldn't have worked...?
Might be worth sticking a few bids on other Profiles auctions before hand, where the final price won't leave you holding the bag.