We are running into this more and more , We tell the customer up front that the labor will be increased by 40% if they supply the part , we also tell them that our hoist time is $2.00 per minute if the part is wrong and they have to go and get another part. Also there is absolutely NO warranty for any labor. This stops most people from wanting to supply their own parts. There is a few exceptions like someone that broke off a spark plug or stripped the threads out then we will install their spark plugs. I do not like supplying or installing used engines but due to the economic situation I install a few of them , I give the customer a list of which wrecking yards have an engine and let them buy it from them. I also still charge 30-40% more for the labor with no warranty. By us not supplying the engine the ball is in their court if it does not work.