No contract, no problem you did your part. Shelby's deal is very fair and you don't need to worry about the car your buying. 13,000 miles is nothing for the GT.
From my perspective, if you really want a GT, find the one that you like best and buy it. Don't over-analyze the numbers or the marketplace. As they say over-analysis creates paralysis.
When looking at that chart of recent sales you gotta think the sellers would be very unhappy on the red and yellow cars only netting them "teens" respectively.
Went online to get the US compliance certificate for my Canadian car. Came back in printable version by email within a couple of days. Cost was minimal, I seem to remember <$50. So what's left? Ownership by email or fax should take minutes. Sounds like you're being done.
Check the forum for Shelby's reputation.
I thought the same thing when I bought my black car, I paid 25k over what they were going for just a few month before I decided to buy but I wanted a black no stripe car nothing else would suffice,
good thing about purchasing from shelby is that you know you are getting a good car, although I did not purchase mine from him I have come to know the quality of cars he deals with is on the upper tiers of cars out there.