Why do you think the frauds did not sell Jim earlier?

I believe the Duke and King, the fraudsters who kidnap Jim, don't sell him so they can use him as bartering power. Their deceit has been found out, in part because of their weak attempt at forging an inheritance, and Doctor Robinson has found them out and tried to expose them.
They lock Jim up and presumably are planning to use him to trade for a ransom of some sort to ensure their financial independence and escape. If they sell him, they will get a sum of money which will of course benefit them, but since they are in a precarious position, trying to con everyone, they probably think it would be wise to keep him hostage in case their plan sours so they can use him to secure their freedom in addition to a sum of money.