buying online totally blows, unfortunately its kinda a necessary evil as local stores , if you even have a local store, ternd to have little or no selection, and want to charge ungodly markup fees.
Couple things. Get a money back guarantee ahead of time in writing. DONOT tolerate restocking fees. COntact vendor/ saddlemaker and ask this to be waived before you order. If they wont, make sure you let them know you are shopping elsewhere specifically for this reason.
Used or private sale ? It would have to be really really cheap, like no more than $200. And that's still a big if. People are basically liars and or idiots when it comes to selling stuff. If they don't have a business reputation to protect good luck getting your money back when your saddle shows up a different size than advertised.