If you're happy with a car that won't cert for anything quicker than 7.50, make the sale dependent on the car passing cert. If the guys not interested in that, walk away.
There's nothing extra to "go thru" for a legal mild steel car if it was built to cert. And it could make a nice car for a guy that just wants to go out and have fun.
Is this a bare chassis or is there a pile of parts that may sweeten the deal? If there was a bunch of usable parts (front end, rear end, steering, etc) and the price was right, it might be worth building or having built a certifiable chassis.