From what I have read, it seems Americans pay much more for used bicycles than people in many other countries. Americans consider their prices to be normal. Maybe a factor in this is that many people take bikes to bike co-ops, whereas in other countries they just sell them cheap.
I think you're mixing up cause and effect. Bike coops aren't a big enough factor to affect the price of bikes in the US. I think what's primarily happening is that the coops are filling in a niche that neighborhood LBS can no longer fill profitably.