[cmckay]

A better way (in my opinion) to ask the question of whether or not your business model is sustainable/ethical/scalable is to ask the question: are you adding value? I think in this case, the answer is clearly yes. Dealing with freelancers (and especially international freelancers) can involve a lot of time and effort; you're taking care of that end of things. It's clear that your clients could go out and find and hire their own freelancers, but that involves work they would rather pay you to do.

As far as scalability is concerned, consider raising your prices. Don't think of it as rent seeking; think of it as providing a valuable service (managing a team of outsourced talent), and price accordingly.