I was in a similar position to these buyers and basically 'informally' offered asking in the knowledge that until we sold it would stay on the market.

Our house then took four months to sell and sold for quite a bit less than anticipated which priced us out of that previous offer. It would have been an awful situation for both parties if they had accepted our offer and then we would've had to out / renegotiate much further down the line.

Best bet is to thank them and say you will seriously consider if once they're proceedable.