I presume we're talking about a regular sale here and not one that went to auction.
As above, should be spelled out in the contract, but from my experience, it's not a lot more than the advertising fees that are likely paid up front anyway. Most contracts also have a clause that if you sell within 6 months of terminating the agreement, that their commission remains fully payable. So beware of that if you're looking to cut them out of a deal.