I think what would contribute to the resale is what exactly is so different about your horse that makes a saddle hard to fit.
I am speaking about western saddles, but all of our saddles are custom built, BUT not to specifically fit a certain horse. Western trees vary greatly between styles, bar angles and even vary between tree makers.
You may be able to find something pretty close to fitting, dependent on your issue, and take it from there. A good saddle maker will help you do this, but not talk you into a completely custom saddle that will only fit one horse and cost you a fortune.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.