I haven't got any imposters yet in the RuneScape Market yet.
For Steam, I had like 2.
What I always do when requesting PM is this:
Code:
Send me a PM on Sythe.Org
Your RSN:
Your Skype ID:
My Skype ID:
Your PayPal Email:
Paste 5 lines of the conversation:
With this, you have enough proof to get them banned from Sythe.Org. If they try to do a chargeback, you have their PayPal Email that they use to send you the money and you can use that as a proof that they have confirmed they are using that email to send you the money. If it doesn't match, you would automatically reject or just request for another PM asking him to type in the same PayPal Email he use. If he refuses, you can just refund him the money.