Sometimes before they will do a repair they will make sure that YOUR purchase is in the computer. For me a bag has to be minimum 25% lower then retail for it to make me even consider buying it preloved. Otherwise I rather buy it in the boutique.
I also set a benchmark for myself, but I factor tax into that equation. In Chicago, it's higher than 10% sales tax. So a 15% discount plus sales tax makes it worth it sometimes.
Not that I've ever found anything... but that's my rule.