phamENG, I highly respect your contributions on this forum. It's clear that you have a lot of technical and business acumen as it relates to structural engineering. But, I strongly disagree with you on this topic. I know I am probably in the minority on this issue, but I have arrived at my position over the last 15 or so years of operating as a small time solo firm. Overtime, an obvious and distinct pattern has been revealed in my practice, and that is that the only time I really face this decision of whether or not to contract with an unlicensed entity to provide pass through services, it is inevitably a fringe element that is basically swimming in the deep end where they have no business and they need me to be their rescue swimmer. Why would I do that? It is like the ring master working for the clown.
Your example of an unlicensed home designer is a perfect example. I face that situation routinely, and I always simply bypass the middle man and contract directly with the owner. If there is any pushback on that arrangement, then one or both of the other parties is either, at best, unsophisticated, or, at worst, shady, and I want nothing to do with the project, and walk.
Also, relying on vague concepts, such as the $$$ magnitude of the mark-up on the engineering services, to distinguish between a passthrough/broker/finder fee/kickback type arrangement and a legitimate "value added" arrangement is too subjective and too easy to game or manipulate. A broker can always make a value added argument for their services, that is after-all, the only service that they offer, connecting two parties that needed help connecting. But, it's my understanding and interpretation of the engineering laws and rules in my state, that brokers are not allowed to sell engineering services, and engineers are not allowed to pay brokers to sell their services. I know that it is done, but it shouldn't be.
I'm not preaching to you or condemning your personal business practices, because I do highly respect your opinions, but I personally see more good than harm in taking a rigid approach to this issue, and vice versa.