You can retract the bid, I believe if there's 12 hours or more left. If it's under 12 hours, then you can't.
The bid retraction is noted in your feedback. If you are the winner and back out, you'll probably get negative feedback from the seller. I don' t know how important it is to you, but as a seller myself, I don't like to sell to people that have too many negatives. As a buyer, I don't buy from anyone with negative feedback unless they have a huge rating and a very small amount of negatives.
I would suggest emailing the seller, using the ask seller a question option. Are there other bidders? Maybe they can just try to do a "second" chance option and offer it to them if you are indeed the winner.
I buy and sell all the time so if you have any questions let me know and I will see if I can help you.