Does Denver have Bike Swaps in the spring?
We used to have a couple each spring in Park City and they were a great place to get used bikes. They were fund raisers for the local junior programs. They were held at a bike shop so each bike got a once over before being sold.
I sold a rigid and an old hardtail at the PC swaps, I got garage room and a couple of hundred bucks for the two of them and a couple people got great rideable bikes for a little over $100.