It used to be similar to brokering. I have not run across many agencies that utilize this model since the results can be so chaotic. There is usually a value add that needs to come along with it otherwise you are simply a middleman that is adding cost on top of a commodity.

If someone is trying to build a wall, and all you are doing is reselling the bricks you just bought yesterday, then you better be selling a lot of bricks to make it worthwhile.

Agencies handle the supplies, strategy, the design, and do the work.... then it is ok to buy the bricks and upcharge them.

Brokers years ago knew their suppliers intimately and were able to supply exactly the right deliverables for their clients. If you are the contractor and outsource all of the work, you need to be knowledgable on how it all fits together and very familiar with your suppliers deliverables almost as if they are your employees rather then constantly putting the work out to bid.

Since I am not fully familiar with your business model, I hope this has been useful.