100k cobra for 20k is way too much unless it is heavily modded or with a rebuilt motor and trans, your offer was very reasonable he might not get an offer like that for awhile. I'd rather spend 2-4K more for a termi with half the miles cause we all know it's a lot easier to sell a low mileage car than a high mileage car of any model. Don't let nobody insult u and take your time and find the right termi u have the power with the money. I would text him back and offer him 3000 less and show him a pic of the cash just to get his blood boiling.
To me, I'm with you on the $15-17k offer being more than reasonable. If the guy is that much of a dick, imagine asking him to take the car for an inspection and a compression check! I guess I personally would look at it this way.... You can find say, a SB car with say half the miles for $22-25k you'd be better off spending the extra coin on that, just from a resale standpoint.