IMVU gives permission to a limited number of people to be registered credit resellers. You can usually buy credits from them cheaper then buying directly from IMVU. Most will also buy back credits, but there are usually some restrictions, such as being a developer,  mimimum/maximum amounts to purchase, how long you have been registered on IMVU.

Rates fluctuate daily and are competive - but someone offering to sell at a much lower rate has to be a scammer.

Note that buying credits from anyone not recognized from IMVU can cause your account to be terminated. Purchase of credits from anyone, even registered resellers is at your own risk and you cannot expect IMVU to help you.

CAUTION there are a large number of fake credit resellers that have web sites that look very real, who will take your money and never give you any credits.

The following list is not complete and not guaranteed - take due diligence and check the IMVU forums for more info.

Suggestions for other credit resellers can be posted in the forums here