Of course he wants to introduce such a clause - a possible onsale at �550K makes the �500K that he achieved for his friend look less than clever.

You could understand a vendor not wanting the hassle of more viewings after having agreed a sale but such properties are often empty and, properly controlled, further viewings hurt no-one.

You could see how such a clause, which serves only the agent's self interest, could prejudice the client's best interests by putting off certain potential purchasers.