Yeah, I wasn't able to make it to the last two auctions(and I'm probably not going to make this one, grr!) when they did have some Rebels, but you're right they did go for a about $600 each. That's still way less than new($3K) or what you would pay on Craigslist($1K).
Actually, I'm fairly certain they've never been out of the parking lot. They only use these for the beginners course(advanced course you have to provide your own wheels) and we never went out of the parking lot when I took it.
7 miles/weekx35 weeksx20 years = 4,500 miles, that's about right.
So bascially they've never been ridden hard but I'm sure they were laid down at low-speeds on a regular basis.