@Commander Vermin
You have to become an established member before you can sell credits on the market.
You become an established member of the IMVU Mafias by posting daily on our forums for 2 weeks.
That will grant you basic seller privileges; at which point you can sell credits.
After you become established, talk to
@Datamine
about getting advanced privileges via IMVU Mafias VIP. With VIP, you can post ads that last longer, which gives you a greater chance of your ad reaching the front page of the black market's credits category.
You can also raise your trust level on the market via your customers leaving you karma; which tends to result in more sales as the community trusts you more.