Exactly how are these proxies come across is what I'm failing to understand. If they were private proxies they would be up almost 100% of the time and would never die. You claim you don't know how long your 'private' proxies will last so they must be mass scanned or scraped somehow and therefore they would actually be 'public' proxies that can be accessed by anyone who comes across them.
What percentage of these proxies are US? Is anybody using these to post on craigslist?
I have been testing our proxies against the CL servers and have seen great success. I have done account creation as well as posts, to verify the proxies are not banned or blocked in any way.