Selling: Ship with insurance, photograph the process of packaging and handing it over to the post office (I use FedEx--it is competitive with the postal service and has proven to be more buttoned up). FedEx is great about verifying an address to see if it is legit. Take a photo of the FedEx receipt and email or text it to the buyer--it'll have a description and a tracking code. Putting in an insurance claim with the USPS is a nightmare.
Buying: Check multi sources on a particular seller. eBay, LL, GetDPI, and Fred Miranda. Ask if there is an original receipt, where the was bought, and get a picture of the item with its serial code. It is possible to track down information on big ticket items such as PhaseOne and Hasselblad equipment. Those items are sold through dealers.
I've had great experiences buying and selling on LL and Fred Miranda. I stopped selling on eBay due to the 10% surcharge.