I had this same problem, I built my trade depot and had metal, plastic, alloy, freight, and oil deposits. The trade depot only allows you to have 4 so you need to build a separate trade depot to sell the others or just live with one being full all the time.
I decided to build an electronics plant manufacturing processors, but when that happened the recycling plant couldn't keep up with alloy production and importing it was more money than I was making from processors.
I ended up having this huge industrial city in the end, couldn't zone any more residential, but my city tanked because of lack of workers. The size of the map I noticed in my first city really hurt my ability to create an industrial city.