on 04-28-2013 11:22 AM
I've had a couple of buyers email me after they placed a Best Offer and explain that they were mistaken about the item and would like to withdraw the offer...all I had to do was decline the offer. Case closed.
Just email the seller.
on 04-28-2013 11:20 AM
You don't. You are expected to pay attention in the first place to keep things like that from happening.
You may message the seller and explain the situation and beg him to reject your offer, but he is not required to follow your request: he can just let the offer expire or he can even accept the offer.