This really isn't a bad idea, as I've seen it work before in another MMO (I see someone has mentioned PLEX in EVE), but I do have trepidations about it.
My worries would be:
1. What if an account gets hacked and someone steals that item from you, and then they use it? We'd probably have to develop a policy in CS about how to handle such a thing happening. It's never a matter of "if", but a matter of "when".
2. Implementation and possible abuse of creating those items (duping). We all know that has happened before.
3. There also might be some legal issues with selling game time for in-game money, but, well, I don't even know where to start with that so it might not even be the case.
Either way, I'll still pass this up the chain if people are interested. It seems that some people would like the ability to pay for their game time with credits, and I'm not wholly opposed to that idea. I've been in that situation where I have a huge pile of in-game currency, but that won't pay for my sub! :P