Ingame Code Selling

[Cyberz]

I am kinda new at this code selling thing. After I recently bought a code (with real money) I sold it to at a much cheaper price (80K). After selling it I realized that the going price is between 130K to 180K. After selling it, the customer came back to me and said that the code wasn't working, I tried activating it for them but the code said that it had already been activated. Out of frustration, I sent TeamMnF an email and they provided me with a new code. So out of curiosity, I have 2 questions.

1. What is the Lowest Price I can sell Prem Codes for while still making a profit?
2. Is there a way I can test the code out myself before selling it to someone? (Just so I know it works)

Thanks