If possible, make the deal certain to a number of provisions. Make one of them that it is not listed in any black list/orbs/rbl etc.
An offer to purchase isn't a contract, however I don't know sedo's system and the way they do things. In their case it might be a contract due to the steps you have to take.
It is normally IP's that are blacklisted. If some joker of a blacklist site blacklisted a domain they will usually remove it after showing the change of ownership.