What type of wholesaler operates mainly in bulk industries like lumber, coal, and heavy equipment

The Answer is Drop Shippers.

Explanation:

This is because drop shipper don't handle the product themselves. The work as only the middlemen.

When they get the order, they chose a manufacturer and after the product is available, manufacturer delivers the product to the customer. The drop shipper only sells the product for a profit under their brand name most of the time.