In England and Wales, no party is bound till the exchange of contract of the sale transaction.
Before exchange, parties can back out bearing any cost they have incurred in the process. However, once you have exchanged contracts there is no going back - both buyer and seller are bound to the deal. If they back out after the exchange there are financial implications of such action and a party can file an injunction and force other party for the completion of the sale.
At the exchange of contract, buyers pay a deposit of 10 % of the transaction cost usually,(unless otherwise agreed in the contract) and remaining cost at the completion.
If the buyer backs out after the exchange, then the seller can retain the deposit and claim any cost, losses from the buyer, or can file an injunction for the performance of the contract ( completion of the sale)
So, if the buyer backs out after the exchange, you can retain their deposit and if you pay your agents, you can recover that fee from the buyers as well.