Charging for virtual cash becomes gambling in most countries, and thus you need a relevant state gambling license. It isn't worth it...
The moment you charge for virtual cash, is the same moment you stop being for "entertainment" purposes, and become classified as providing a gambling service. I looked into this surrounding sites that charge a "signup" fee as well, and they do that legally as a guise for membership and not the virtual credit they give you to start with... even though basically you're paying a membership fee to obtain starting credit. They do however then provide free methods of obtaining further credit by participating in their community and they don't sell it directly. Those big sites who do sell credit to bet with, have gaming licenses.
If you sell virtual credit for betting purposes, you open yourself up to be reported and criminally charged for illegal gambling. I wanted to do it for my sites... and quickly found out the consequences without a gaming license.