Credits are essential to using IMVU. It’s the in-game currency. The average user would mostly use credits to buy items in the catalog. You would also need credits to buy badges, art in the forums, custom products from creators, and even a creator has to pay to publish a product in the catalog.
Usually when I would buy credits I would buy a gift card at Walmart. A lot of the time the gift cards would give you extra perks, like some new furniture. I liked the gift cards because I had a few alt accounts, and with the gift cards you could get the extra perk of removing the Guest_ from your username which made you look like a noob
I'm not sure how much credits are actually worth or how the price has changed over the years. Teenage me didn't really care, I also spent a lot of time doing surverys/offers to get credits for free. IMVU does credit sales pretty regularly on the classic website. The sales on the classic website don't seem to be on the mobile app though.
(This is what credits cost on Jan 14, 2025)
It's pretty wild to me a fairly simple avatar is some where around 10,000 credits which is $10.

Credit Resellers

Something that exisited when I joined IMVU were authorized credit resellers. These were websites that you could purchase IMVU credits from for a cheaper price. More importantly developers were also able to sell their credits to the reseller for real money. I'm not sure how lucrative IMVU income was, I've heard success stories about SecondLife but I don't know if was the same for IMVU.
I do remember buying credits from resellers. You would pay credit reseller on their website and then their IMVU account would send you credits. DevCredits was a popular credit reseller that still has an website. Most of the credit resellers sites are no longer active. I do remember seeing IMVUX and First Meta Exchange a lot during that time. I remember there being a lot more I just don't know what they were called.
FMX in November 2010 DevCredits in July 2011 IMVUX in August 2011