I worked in the customs, and I heard that this work is haraam because of the mukoos (levies), but I continued in this job for a while, because I was hesitant. Then I made up my mind and left the job. Now I am wondering, is it permissible for me to benefit from the experience acquired in this job to seek provision by means of it in a permissible job, such as being employed by somebody who works in import and export, so that I can take care of clearing the goods in customs, which are permissible goods. In other words, is it permissible to benefit from experience which was gained through a haraam job in a permissible job?.
There is nothing wrong with you benefiting from this experience and using it in a permissible job.
Similarly, if a person deals in haraam things, and benefits from that experience in business and handling money, then there is nothing wrong with him using this experience in permissible jobs. Islam brought the command to give up riba, deceit, cheating and other haraam things but it does not forbid any of those who have repented -- or even those who have not repented – from working in permissible fields benefiting from that experience.
As for working in the customs, the ruling on that has been explained in the answer to question number 39461.
We ask Allaah to accept your repentance, make the consequences good for you and to grant you provision by His grace.
And Allaah knows best.