To make sure you can always take back the PSN, you need atleast 3 of these things. Name on the account, address, creation serial number, DOB, and last 2 transactions made. To be safe, buy 2 cheap things on the marketplace and write them down on a notepad.
With this being said, its also really easy for the person that sold it to you to take it back as they most likely know all this info too. Make sure you buy from a legit person and make sure you know atleast 3 of the things I listed.