Having aquired an iPhone 4 (yes, it's awesome and no, it hasn't dropped any calls), I wanted to sell my iPhone 3G 16GB. It was in pretty good condition, having been covered by an InvisibleShield most of the time.
Starting with the MoneySavingExpert article I looked at the various options listed at MobileValuer to see what I could get.
Based on the terms and conditions and the feedback I read Envirofone seemed the best bet. I chose £158 cash instead of Argos vouchers. Today, Envirofone is listed as offering the second highest amount of the 10 options. I don't think they were that high when I made my decision.
2010/08/08, received an email saying that I had qualified for Collect+. This saved me the postage fee.
2010/08/09, sent using Collect+ at a local petrol station
2010/08/12, received email saying "We have now tested the mobile phone you sent to Envirofone and are delighted to let you know that it has passed successfully, meeting all our terms and conditions."
2010/07/17, full quoted payment received via BACS transfer