I never purchase from big dealers as they invaribaly sell the worst used equipment because they have to take things in trade or purchase heavily discounted and feel they can always sell to the unknowing. There are a few smaller Dealers in used equipment I have pruchased from.
I typically look for lightly used equipment from the original purchaser on eBay, look at feed back and always establish a dialog and for large ticket items a phone coversation is a must. Pay with Amexo and always have items sent insured, with confirmation/tracking # supplied to me. So far, it has worked out extremely well. I never purchase junk as it will always be junk and the extra price to pay for Mint equipment is a small price in the long run.-Dick