Bob Atkins - Buying used gear on the Internet

How to avoid getting screwed when buying used photo equipment via the Internet
Green flags mean "so far, so good"
Yellow flags mean think twice.
Red Flags mean think three or four times. Do you feel lucky?

Get an address and home phone number. Call the phone number and speak to the seller. Check the address and phone number match via an internet directory search. If the seller will only give you a cell phone number that's a RED FLAG! Remember, if you can't track the seller down now, you won't be able to if the deal goes bad either.
Get a work phone number if possible. Some people may only have cell phone service I guess, but any and every business should have a listed phone number. Not everybody can take calls at work of course, but again, if the seller seems evasive, it's a YELLOW FLAG!
Unless you know the seller, get a real email address. All ISPs give you an email address. Some people (myself included) chose to use a hotmail, yahoo or other free email service, but they should have a verifiable ISP email address that they can give you. Failure or refusal to give a real ISP email address is a RED FLAG! Some people do have free ISP service via JUNO or NETZERO, but they are the exception, not the rule.
Do an internet search on the seller's name. Anything odd turn up? Does the seller have a website that looks well established? If the seller isn't found by any internet search engine, that doesn't mean anything, but it's at least a YELLOW FLAG! If the seller has a long standing website, is an active participant in photography forums and seems well know, that's a GREEN FLAG
Ask if the seller will ship the goods COD if you pay all the additional charges. You may not want COD, but if the seller absolutely refuses, it's another YELLOW FLAG!
If the deal sounds too good to be true, and you don't know the seller, then it usually is! This is a YELLOW FLAG! Yes, you can get real bargins, but they are rare. Probably rarer than scams.
Insistance on a cash payment, or payment via some unknown internet service based on "E-gold" or other wierd system should raise a RED FLAG! If large sums of money are involved, consider an escrow service. If the seller refuses to consider an escrow service (even if you offer to pay all the expenses), then that's at least a YELLOW FLAG!
If the seller insists on payment to a P.O. Box, that's a RED FLAG! Note that even a real looking street address may be a MAILBOXES ETC. store or some similar service, so a real street address is no guarantee that the seller actually lives there. Some people (e.g. university students in dorms) may only have a P.O. Box, but again, that's the exception, not the rule.

I've bought equipment form lots of people, including some very expensive items from US sellers and some less expensive items from sellers in countries like the Ukraine. So far, so good. I follow my own rules and have yet to be scammed. Not that it's impossible of course, but I like to reduce the odds of it happening as much as possible!