What is the value of the domain? The value, to the buyer, should not ever change - doesnt matter who the buyer may be.
Figure out the value of the domain and get back to the prospect explaining how it can help him/her with their company, sales, etc. Explain to the prospect that each day that goes by the traffic to that domain could be a potential customer AND if the domain is purchased by a competitor then, well, the competitor now controls the traffic and the potential of converting customers.
A Porsche is the same price on the lot if I walked up to the dealer or if Bill Gates walked up to the dealer. The value of that car remains the same - doesnt matter if I or Bill Gates are wanting to purchase it. Price the domain according to its value, not according to the buyer. Same goes especially for houses, commercial real estate, etc. Where cars are mass produced there is only one house at the address. In this case the value can increase if there is competition. This is another topic but if you can get two or more folks interested, bidding on that name, they may blow through the asking price that you have established for the domain.
Bottom line is, only you can know what the value of your names are - do some homework and price them according to the value they can bring to the buyer.
IMO