I know that there's plenty of people who do award booking services, where they charge a fee (i.e. $150 per ticket) but they use your own miles. For an F or J award with tricky routing or competitive availability, high number of travelers, or if you just don't have the time to do the research this can be great.

What you describe (a broker paying to use someone else's miles, then booking your trip using their miles) sounds like it would be even more expensive than that. They would have to pay someone for their miles, and then charge you for their time. I would imagine that they would want some sort of non-refundable deposit before they tell you all the routing information. Only way to find out, i guess, would be to contact one of these 'brokers' and ask how the process/payment works. If they don't take any deposit, and will tell you the routing before you pay then I guess it might work.

Also suppose they do find something they can book for you, it would also require you to have the miles in your own account to do it yourself. If you already have the miles yourself, I think you are better off either researching yourself or hiring someone to book for you for the lower fee. $150 per ticket for a business or first class international trip isn't so bad.