I might be able to shed some light on this.
Web-based proxy servers get huge amounts of traffic in a short time as its something users want, but many people are unaware of whats needed to run it, they start out on a shared hosting enveiroment and when the proxy gets a fair amount of traffic it will overload the server and normally, the host will kick them off, they use anywere from 20GB-2TB bandwith a month depending on how its managed.
When I first joined the domain industry over a year ago I was one of the only ones in the industry who ran a proxy site, The traffic quickly grew and I got an okay price for it and they started popping up everywere.
It is mainly people who think they can make a quick buck who start them, they get kicked off and look for a sale. Unless you can afford to pay for a dedicated server (or more than one) I wouldnt plan on getting a proxy site.
Potential buyers do not understand that running a proxy site isnt as easy as uploading and going.
I run (i will remove if its classed as advertising) ukproxy.com and several other proxy related sites, ukproxy.com alone takes a P4 2.8Ghz/1GB Ram server with no fancy stuff such as cPanel to run smoothly, I have fortunately limited downloads to 2MB each via the proxy to save bandwith and I expect to use 200-300 GB this month. The site is earning me enough to pay for the server and a little profit, This has took me over a year to get right with owning several proxy sites, I can happily say, I'm not really in a position to need to sell this site.
My advice to you all is unless its earning over $150 a month, your either going to be losing or cutting even and I would avoid it, If you park the site, the traffic seems to last about a month then die out.
If you need any help with your proxy just send me a PM.
On a side note: The main use of these proxies is:
Porn
Downloading Warez (I cut down on it)
Anonymous surfing
Bypassing IP Bans
Social Networking Sites (eg, Orkut, Myspace, Hi5 and others)
Gaming Sites
Webmessengers