You would normally put your house on the market before you buy another house. So you would choose the best auctioneer to sell to your house.
If it happens that he has a house you wish to buy, there is no reason why you should not buy from him.
If you have bought another house first and now want to put your own house on the market. The auctioneer you bought through may be more motivated to sell yours so that both transactions go to completion.