Talk is cheap. What really counts is whether the buyer commits real money to the deal and arranges a survey. So, an important milestone is when an appointment is made for the mortgage lenders’ surveyor, or a structural surveyor, to visit the property.
If this is delayed, it means the buyer may not be serious, or is having second thoughts, and it certainly means that it will delay the issue of a mortgage offer.
So, fairly soon after a sale is agreed, you should be asking the estate agents to find out when the surveyor is coming. At the very least, you need to know that any delay is due to the surveyor being booked up, not the buyers stalling.
If the estate agent can’t get hold of the buyers or can’t get a straight answer about the survey, that is a reason to be worried and perhaps to threaten to put the property back on the market. (This threat is quite effective. You are not saying you’re pulling out of the deal. But you are putting the buyer on notice that if someone else comes along with a better offer you might take it.)