Most of my audio equipment was purchased used via Audiogon, US Audiomart, Canuck Audiomart, and Ebay. I’ve had good experiences thus far. The best way to protect yourself is to use a service like PayPal which offers buyer protection in case the equipment you purchase is not as represented, or if you’ve been scammed.
Also avoid using PayPal Friends and Family, since it removes any buyer protections that regular PayPal offers. Check seller feedback, and get as much information from them as you can regarding the particular piece of audio gear you are interested in purchasing. This should include as many photos as possible as well as a piece of paper in each photo listing the current date so that you know that the photos are recent and not taken of someone else’s listing for the equipment (as part of a scam - this has been happening a lot more frequently over the past few years).