You definitely have weathering skills that are worth charging for. Everyone has their own comfort level as to what they will do on their layout. I happen to like weathered cars and have done a few. While mine are no where as nice as yours I am happy. There is definitely a market for that kind of work.
If this is your 1st experience at selling a service you provide take note of the following gleaned from personal experience. Eventually you will run into someone who no matter what you do will not be happy and will try to stiff you for your work. On the other hand most people, especially train hobbiest's are fair and good people. But you really do have to spend some time finding out what weathering means for them. Your idea and someone else's could be very different. But you can alleviate that just be communicating with the customer.
Ed