Sellers / Traders:
1) Don't be a dick, send your stuff once someone has paid.
2) When shipping, always get a shipping and tracking number. It just prevents issues, and stops he-said-she-said because you have proof.
3) Use paypal, unless meeting in person. Do not mail cash, thats just stupid. You are buying through the internet, why not pay through the internet?
4) Describe as accurately as possible and take as many pictures, there is never too much. If it has a scratch on the google, say "there is a fucking scratch", that isn't normal wear and tear. If it is for you, you are too hard on your equipment.
Buyers
1) Pay using PayPal, don't agree to mail cash. Bad idea. I thought that would be obvious, but I've seen it a few times, and saying it twice won't hurt. If it doesn't show up, you can dispute and get a refund. You can't do with cash. So many safety parachutes, there is no excuse not to use it.
2) Get pictures. As many as possible, you can never be too safe. You don't want to get a pair of skis and notice a huge core shot because they didn't send good pics that weren't took with a fucking webcam.
3) get all their information. Phone number, address, even add them on facebook to call them out publicly if something goes wrong. Hell, if you are sketched out, get one of their friends info to ask what is up.
4) SHIPPING AND TRACKING. It was invented for a reason. Get it. Make sure they don't "forget". That way, you'll know when they ship it, or if they are just buying time.
5) If it is too good to be true, it is. Old idiom, but necessary. If someone is selling brand new FT's for $50, think about it.
This shouldn't have to be here, but make sure the person has actually scammed you before doing a callout, it can really damage someone's rep, so be careful with the power that is entrusted.
Some of these things haven't happened, but in general people are pretty stupid so better to have more than less. Hopefully this helps someone in ordering, or provides precautions. Hope I got everything.
