Is it not the case that the potential buyer to whom you wish to sell will simply pay a refundable booking deposit to your estate agent - amounting to say, €5k - which signals their serious interest but which is paid back in the event that they don't go through with it?
I was sale agreed through my EA to someone who was dependent on selling their own house before they could proceed. The EA recommended putting my place went back on the market after a month and a half because there was no immediate prospect of their sale. They were repaid their booking deposit and I was back on the open market again.
The reasoning was that there was no telling when the prospective purchaser could make their own sale, so I was missing out on potential new interest, sitting at the end of what could have been a very long chain - possibly to no good end.
I would not entertain signing any contract which was subject to their sale unless they were sale agreed themselves or had the ability to complete without the need to sell their own house.
It's bad enough that a sale-agreed falls through but very not good when you have contracts drawn up and these collapse.