Ii sold a '90 Burley Duet a couple of years ago for $450 and have seen others trade at around that number. You should be able to find a safe, comfortable, and significantly lighter tandem for that number. For your budget, or less, you should be able to come up with a Schwinn beachcruiser, which is, at least, a known quantity.
The tandem in the ebay ad has a band brake on the rear. I have some experience with the same brake on very early Dahon folding singles. They are just barely acceptable on the Dahons and I would be very wary of them on a tandem.