Ideally i like to purchase from original owner who has all the original packaging material and manuals.
i find most people in this hobby are fairly honest and want to change their systems occasionly as they get the upgrade bug, I have not had a bad experience yet ( knock wood)
i have been upgrading my YBA electronics form the entry level up to the top of their line and have not taken a huge hit yet on resale every purchase i have made sure that all original packaging is in place then when i go to resell it makes a difference.
if you talk to the seller off line and even by tel if you are purchasing a large ticket item it will give you a sense of the person and their sincerity, but i have found nothing but great feedbackbuying & selling.
Plus if on audiogon or audiomart the feddback process is very good in weeding out scammers.
I have never purchased off of Ebay though so i cant comment on that service, i am leary of buying on Ebay so i stay away form that buying option
Cheers!