I enjoy trading. I also enjoy delivering pizza. One pays me real money, the other ingame money, roughly, per hour, and per opportunity. I can, to a degree, trade the one for another, by taking time off to trade, or playing less to work more.
So, would I pay money for ingame credits? Quite possibly. They're both representations of my time.
Ingame credits do have a rather limited appeal to me over food, of course, but I'm well fed.
I would -sell- my credits for $4 an hour's trading.