I buy used all the time on eBay... Many of what you might consider Internet/brickMortar camera dealers are on eBay as a secondary marketing distribution system...
I bought a used D300 from Robert's Camera thru their web site. It had a 1 year warranty. The switch acted flakey so I called them to make arrangements to send it to their repair folks. They didn't have a used D300 in stock that was the same condition as the one I'd bought earlier in the year... So they simply replaced it with a Nikon Factory Refurb'ed D300 for free...
If you do your due diligence, the same companies and service are available on eBay as any other camera dealers...