Well here is my experience
1. I have bought and sold more than ten phones in either direction, however all the phones i purchased were in a box with an IMEI number and I would go with an affidavit to be signed by a commissioner of oaths saying I had purchased said phone from xxx with ID number yyyy.
I never ever had any hastles at all with these guys and once when my phone did get stolen even though i only had an affidavit i was able to recover it by going to get a police report, submitting to econet and then waiting.
the used phones i never get, usually they have battery problems etc. and from time to time the guys are willing to walk with you to gold tech electronics so you can compare prices etc. The illegitimate dealers will run like fire when you pull out the affidavit.