James, that is indeed a great approach. I like it so much I am going to make it official policy at Green Square Music:
 
If you buy a pre-loved instrument from us, we will give you the full purchase price back when you trade up!
 
(Assuming, of course, the instrument comes back in as good condition as when it went out.)

I'll bet that really helps develop a loyal following.  Shame you're not local to me in Milwaukee...
 
Bill