a web dev agency is really about selling hours while providing the value add of hand holding and managing projects -- the great thing is that you can mark up the services of freelancers, but the bad side is that you're on the hook if anything goes south (in addition to paying for added overhead). so for example if the client goes under and doesn't pay you, you still need to pay your freelancers. unlike a product driven business the nice thing is that you don't need any overhead to get started, however the down side is that the name of the game needs to be pleasing clients rather than yourself more often than not.