Worried about "hurting your relationship" with them? What kind of sense does that make? What if I told you I never understood that reasoning?
When you have a friend that sells cars - you better use them. Heck, you can hold them to being more accountable because if they are a true friend, they will not want to loose you because of a bad business dealing.
If you needed surgery with a Brian Tumor and you happened to be friends with the Top Brain Surgeon in the world, you know you would use them!
That brings me to using friends when they do a service or business you are going to need. If your friend is the type that "presses" every chance they get - "Use me to do this, use me to do that, hey, when are you going to use me, when are you going to buy...", I'd set them straight and would not use them :)
But if they are ethical, moral and don't press you about doing "business" with them and they are good at what they do - Use Them!
So, the next time your friend tells you they cannot use you for your "product or service", let them know that maybe that you are not their friend :)
Be Safe - think about it and we appreciate you patronage :) Besides, don't forget the way in which we do business - It matters.