Just like on the ground teaching jobs; I wouldn't bother with an agent.

Make a website, register a company for tax purposes if needed, do your market research (by yourself) and be prepared to spend a little on advertising if needed (you probably won't need to).

Basically, all the work that an agent does, you are perfectly capable of doing yourself. In theory, you could even expand, recruit other teachers and become an agent yourself!

The one big obstacle other than marketing costs and a little hard work would be the legalities. That is pretty much dependent upon where you operate and there's a lot of grey areas.