As a dealership employee, I'll stand up for them (giving them the benefit of the doubt)...
KBB value for an 07 GT Premium with those miles is over $20k. So they're not overpriced for "retail value" they are right on the money. It'd be different if they said "Oh, look at this awesome custom master piece - only $35,988!!"
Also, I'd be willing to guess that the guy (or girl?) who traded this in, fed them every line that they've typed. I highly doubt a GM dealer looked at the Mustang and knew all of those facts/terms/packages (although some places do it) and I'm sure they didnt search all over the car for that sticker JUST so they could advertise it as a 5.0. Most likely, the previous owner wanted more for his run of the mill mutt-stang and said "I need more for my trade it's got this,this,this, etc etc"
Just my .02 I could be wrong, but I see both sides every day =)