I would keep it on the market until they go sale agreed at least, this would also put a push on them to sell there house. But i would instruct your EA to keep off the sale agreed untill there contracts are signed. We were in this situation last year and they had gone sale agreed, this fell through, so we waited as they asked and when they did sell there house, they had bought another house cheaper behind our backs while we were waiting for them...so be very careful, especially as they can pull out any time up to signing contracts so best of luck and hope it works out..
As an aside, i would also check if they have mortgage approval for the balance that might be needed to complete the sale on your house, incase the sale doesn't cover the full purchase price.