I've been looking for used panels on places like Craigslist, and found a set that would fit well on the flat shade structure roof I have available -- 6 4x10 Heliocol panels, 8 years old, no leaks according to the seller, comes with connectors and brackets. There's no asking price, just "make an offer", so I'm trying to figure out a fair range given their age out in the sun etc. My first thought is wanting to start in the $500-$600 range, but I don't know if that's way too low or way too high. Any thoughts?