I have sold my phone on eBay. What should I do before I send it off? I have an Android phone which I just sold on eBay. What measures should I take before posting it off to its new home? Please answer thoroughly!
As said already, factory reset for a start. 

If you care about the privacy about what is on the SD card you should use a secure erase utility. Even after formatting the card you can still recover the data...

If you google for secure erase you'll find a bunch of freeware tools to do the job.