https://geekhack.org/index.php?topic=66013.0This thread might help.
Or this one
https://geekhack.org/index.php?topic=58159.0for r/mechmarket
https://www.reddit.com/r/mechmarket/wiki/rules/rulesAnd honestly most things are common sense and doing your own research.
I've had plenty of deals both ways that seemed shady but so far most things have worked out ok, there will always be idiots.
Heck I just got 2 keyboards recently that I didn't even get asked to pay for until after I received them, this used to be the norm but too many shady folks nowadays.
Personally I tend to lean towards using a paypal invoice and always pay or sell as goods, and even when trading if you aren't comfortable agree on a value and cross invoice with the other person and pay each other that way you've got some recourse if things go bad.
Unfortunately there are people who know exactly how to work the system and scam, this can not totally be avoided just be smart about what you do and cover yourself as good as you can.
I like to put a note when selling something that the buyers need to understand what they are buying the same goes when selling you need to get your head wrapped around it so you understand what to do, once you get a few things sold and delivered you'll get the hang of it.
Ask for feedback from people, use trackable shipping, if it seems to good to be true it probably is, follow the rules and as much as these types of threads get old at the end of the day just ask and we'll probably help.
PS: many of us who have been around a while don't mind the occasional PM either if someone needs some direction
PPS: I like to ship through paypal using USPS either first class for smaller things or priority for other things, flat rate boxes are often your friend. Use tape and make sure to use padding and it is super easy to print labels at home as well.
PPPS: I'm not responsible for the contents of this message and do not officially or unofficially recommend or endorse anything I said, this information to be used for entertainment purposes only. Don't try this at home. If you poke your eyes out it's your own fault.
PPPPS: I've had much better luck buying and selling on r/mechmarket than I ever have here.