Frank,
You stated in the original post that this unit has never had any tunes uploaded to it. So did you just buy a "generic" (for lack of a better term) XCal 3 with nothing but the "canned" tunes loaded on it from SCT? If that's the case, then I don't think there will be any particular ECU code tied to the tuner.
Maybe I'm wrong, but I thought the ECU code was loaded by a custom tune provider like Torrie as part of a custom tune.
Nevertheless, I agree with Eddie & Carey. Your XCal might have a better chance of selling if you lower the price, given the fact that a buyer will have to buy custom tunes to make it work with their Trac.
About the only way you're likely to get your original asking price is from a Mustang owner who wants it for just the "canned" tunes.