I'm not sure it's clear that you should rescind.
The 50,000 points are worth between $500 and $1,500 depending on which TUGger's calculations you use. That narrows the effective price difference. Thus, from a value standpoint the logical question is whether the opportunity to exchange the use of your week for Marriott Rewards points, a privilege you enjoy by buying from Marriott, is worth that remaining difference. Only you can decide. It would be well worth that price difference for me. I’m guessing that this forum’s points guru, pwrshift, would buy a week from Marriott with such a small price differential (versus resale price) in a flash!
A couple of additional points....
If you saw a resale advertised for $6,900, it's almost certain you can buy one for less than that, because asking prices for resales are just that - asking prices. Negotiation is the game to play when it comes to resale purchases.
It sounds as though you are pretty sure that you want a Mountainside Gold. Great! If so, I believe your simple choice is as I stated. If not, then I would echo Cindi's and Stefa's recommendations to rescind.