Can our buyer representative's negotiate a house that he listed?

No, he cannot. He is now what is called a 'dual agent', and cannot to the advantage of EITHER side, should you continue to use him as your representative. My state has just started what is called 'designated agency'. In a situation such as yours, the managing broker of the agency can ign (or designate) another agent from within the firm to handle your interests in this particular property. Ask your buyer agent if Tennessee has 'designated agency' or similar.