Looks like a cool boat and the price seems reasonable, there aren't many of those around so nailing down market value might be tough. They put those mercury jet motors in a lot of boats in the early 90s, I would read up on them as some models were lemons I believe. Some models had a problem with a plastic shaft on the oil pump breaking and seizing the motor.
The guy says he doesn't have time to put it in the water, I'm assuming he means you can't test drive it. That is a big big red flag. You probably should test drive any used boat, you REALLY should test drive an older jet boat. Can he wife or brother go with the for the test drive? Let us know what happens.