I entirely agree with @markhallva – I would be extremely careful to only take what I can deliver myself. People are wanting to buy your services, not someone else’s, and if you are buying the same service for less than you charge yourself, the question is why they are cheaper.
You should then end up spending a long time checking the quality of your supplier’s work in case it is lesser than your own. And if you’re going to be tied up doing that, then you might as well just do the work yourself and have the assurance that it fits your brand and offer.
Some people do operate in teams here, and they do it very successfully, but they are highly experienced and I feel it takes a long time to get to that level and to be able to hire someone who will deliver as well as you do on your own.
I have been copy editing here for 2.5 years and only now am I beginning to consider help from other editors. However, my first port of call was to employ someone who now works with me side by side in my office, every day–so that we truly are cohesive. I still only use him to double-check my own edits and would never give him a client’s job to complete.
I still have room for a freelancer but it is very hard to find someone whose standards are high and whose work would still give me a good operating margin. That’s the issue. Good people cost good money–rightfully so!
It is your name, your brand, that’s at stake–and clients deserve to get the name they think they are buying. So, as Mark said, if you are buying in services, you need to let the clients know. While there is nothing to stop you doing this on a practical level, you need 100% confidence that your supplier delivers exactly the same standard that you do, and that clients are happy with you using help.