You would be best to simply hold on to them. When you go to sell the Mustang, put them back on and sell the Borla, as they will sell.
There are plenty of stock axle back mufflers for sale on Ebay, and ocasionally in the classifieds here; I never see an offer made. At best you might get 50 bucks for them, but the shipping would come close to that as well, and would kill the deal.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.