Well has the buyer signed the contracts yet?
And have they paid the non-refundable 10%?
Usually the buyer signs first and then you do, and then you agree a final closing date.
If they have signed, then chances are the sale will go through, the buyer is just stalling for time to suit themselves -whether waiting for their mortgage to come through or something, you never know really.
It is a nerve-wracking process and it's never over till you get the call to say the funds have been transferred.