I find this method one of the easiest and thought I'd share it here. I have made at least $15-20 profit with each proxy website I sold. This is what I do...
Purchase a .info domain name from godaddy (costs less than $2)
Purchase hosting for a month ($5 with 3-4 sub domains)
Download the PHProxy script and put in my google adsense pub ids.
Upload the proxy.
Promote it using free methods - directory submissions, yahoo/google groups, link exchanges, topsite submissions.
Sell it at the end of the month for 5x the revenue made from adsense. (For example if I made $10 from google adsense in a month I would expect a decent $50 for the proxy).
Sell it at forums like sitepoint/ digitalpoint.
The total investment is about $7 which can be earned back by adsense. So the entire sale price is your profit. I have done this for so many proxies and not one has ever been sold for less than $20. Not a lot of money but for 15 mins of work a day and $50 a month from just 1 proxy seems good. Now imagine you have 5-10 or 15 proxies. ;-)
I would be happy to help or answer any questions on flipping proxy websites.