Thanks for the replies, guys. The boat itself is in great shape; the shells are very strong, and theres very little flex in the boat itself. The sail looks fine (it felt strong, at least) and it has all the little things, including a road worty trailer with new wheels and bearings. The only real thing it needs is a trampoline, and those look to be quite expensive (he pulled the price down to 500, he's been trying to sell it for half a year), so a new trampoline could easily add half the cost of the boat to itself.

I'm guessing that using the lawn chair webbing is a bad idea, because it needs holes in the pipes. Would it be a good idea instead to create a "trampoline" somewhat akin to this, where it's attached to the poles via loops, or would that not be strong enough? I plan to be gentle on the boat, and not jump, so I pretty much just need something to sit on.