Re: How much is a '69 Plexi Super Lead worth?
If the front and back panels look clean, the amp doesn't look abused, and there's no visible holes from mods, then see how low you can get it for.
If he knows you're serious about buying it, he shouldn't have a problem with you pulling the chassis out to inspect the circuit. Just look at all the solder points and pots and trannies to make sure they look old and original. Even better, if you can find a chassis picture on the net somewhere, to compare.
Keep in mind, most people sell early Marshalls for one reason. They're so loud, they can make you sterile!
If his price is less than $1200, and it's a clean, I'd jump on it. If he knows it's worth much more, I'd pass.