Its worth whatever someone's willing to pay for it, like anything else.  I know that's a cop out answer, but consider this: a guy who has a '65 GT coupe that's just been in a wreck where it's totaled.  If he wants to resurrect the car by rebodying it then its worth a lot to him, maybe.  On the other hand, he could really just buy any '65 coupe body and use his VIN tag, etc. and make it into a GT.  So, I think the seller thinks the GT in the title makes it worth more, but in reality I don't think it does because there's no difference in the body GT to non GT.  Were it a '70 Boss 429 body that had all the mods for the 429 engine, or a Shelby shell with all the body parts then it would command more than its mundane counterpart. 

Given the amount of work required, and that its the least desirable body style, I would value it at maybe $2,000 if it comes with a clean title.  If there's no title then maybe $1,500.  You're going to spend A LOT making that into a complete car that can be registered and driven. 

IF you can get it for less  than $1000 it would make a fun resto driver,,,  new full frame/suspension with modern engine $15k to $25k ,  paint interior wires $10k to $20k  about 2-3 years less if your one of us old farts but we don't work that hard,, you can will have a in-creditable driver worth about $30k-maybe a little more   BUTT build it to drive the snot out of it and then sell it,,   ME I keep the factory GT trim just because,,         

Well, if you wanna make a handsome profit, buy it for what others suggest you should be able to get it for, and drag it up to the midwest.  You will make a tidy sum.  That shell (if truly rust free) is exactly what I was looking for a couple of years ago, and couldn't find. period.  Jim Bowie was searching at the same time, and bought his Fairlane.  No decent stang shells to be had, nowhere.

In that case, put it on eBay.  It will bring highest dollar there.  Just be clear that the buyer is responsible for transportation and be clear as to where it is.  I'd start the bidding at $1,500 and see where it goes from there.