Depends on what you need. I work in sourcing now and while most of the time we deal directly with the manufacturer there are times we have to use trading companies. 

As an example we have a new client who wants to buy sanitary ware (toilets and basins). Problem is the factory MOQ's. They want a minimum order of a 20 foot container of each product. Our client wants to buy 20' or 40' containers of mixed products. So we're using buying from s trading company who can supply us with lower volumes. Of course we know we pay a premium for this service but our customer is happy with the prices as long as we check the quality before shipping. 

I used to buy all my vocabulary through an agent... but it was always shoddy work! Missing letters, inconsistency, and total confusion.. and what's worse, when I complained that the readers of my vocabulary couldn't understand - they attacked me!!!

I'll never go through agents for my words again!

I've used both manufacturer and trading companies.
While you can get a good deal directly with a manufacturer the service can be the down fall of the purchase.
Trading companies by large have people that are more keen to close a deal and sell you other products on their books.
They also are good organizers of travel and accommodation and visas if you want to see theses factory s .
Also thier English is good like spongebob and Vicki's is
But you pay for the service.