I value my good reputation both on MiceChat and VMK. Feel free to ask any of the MiceChat moderators about me.
The reason I prefer to receive the credits first is for my own protection. If I PM the code and it is used... it cannot be traded back. Trading codes for credits can be the riskiest type of trade because it's so very easy for either party to scam the other.
It's possible that I could PM a perfectly GOOD code... get the credits... then be told the code was faulty and asked to return the credits. The "danger" works both ways.
I'm sure there are plenty of people willing to make this trade. If you are uncomfortable with my conditions, I understand.