I would email sedo immediately and see if anything can be done. I don't know if that will make a difference as the buyer is notified as soon as you decide to sell .
Yah, I would agree with what mejcdj said. I don't know if they will say anything. They will probably just copy and paste a section from their 500 page TOS.
pm sent. good luck. you may get banned from Sedo if you back out... but perhaps the domain is worth getting banned over ??? That's your decision to make.