I've bought from KEH a couple of times, they are solid.
I've bought and sold on eBay, if you choose your seller carefully you should have no problem.
In the end, I've done my best on craigslist. Some folks have all the money they will ever need and sometimes they just want stuff to go away.
Picked up more than a few great deals at thrift stores, antique shops and pawnbrokers too. Local is completely random but sometimes it can be amazing.