How to avoid the known Scammers (ban evaders, vaders, evaders)..
Okay, so let's assume that a user contacts you on Skype about your sales, services etc. First thing you wanna do is to ask him to send you a private message here. Then look up users skype name (right click and View Profile -option).
Open Google and search skypeusername skype. Browse through the links that relate to Runescape blackmarket (OSBot, Sythe etc.)
If you find that user who is using that skype name is banned from any of these sites, or have a open dispute against them, I would suggest you not to deal with this user.
How ever, if the result is clean for Skype name, this is what I personally like to do, when the trade or sale involves PayPal, ask if they are verified in PayPal, and then ask what is their PayPal email. Again, same thing - Google search the PP email and look for same kind of results as in the first step.
If the user has not verified their PayPal, once again - I highly recommend not to deal with that user also.