Credit Resellers are users or companies that buy and sell IMVU credits, usually cheaper than IMVU does. You can buy credits from them or from IMVU. If you want to sell credits, you have to deal with them, since IMVU does not buy credits. (Note that you need to have a lot of credits if you want to sell them, typically 50,000)
 
It is crucial you only deal with IMVU authorized resellers - there are hundreds of fake resellers that will take your money and you will never see the credits, and IMVU cannot help you.
 
This IMVU list, though outdated, has many fraudulent resellers listed (June 2008)