Actually it's federal and state laws that are involved.

You can call it a "commission for non-employee" if you want, but that doesn't alter what it is.

But, besides the legal issues, you create another dilemma.

If you increase your prices so as to allow for a "commission" to the builder, then your pricing is out of line with the market (you're higher).

In this case, if the homeowner gets another bid on his/her own and discovers that you're 10%, 15% or 20% higher than others for the same products, the homeowner perceives this (rightly so) as you trying to overcharge for the product. Ergo, YOUR reputation is damaged.

Then, additionally, the homeowner takes the builder to task for referring them to a company that overcharges. Ergo, the builder's reputation is also damaged.

You not only lose that particular job, you very possibly end up losing that builder. It's a NO WIN situation.

How to address this?

The proper way to have the builder compensated, is to have the builder specify to the homeowner that your products/services come under the builders "cost plus" addendum to the building contract. The builder marks up your products/services as the contract allows, and the homeowner is aware of these charges.

In this case you don't seem to be out of line price wise, the builder is doing business "as usual", and the homeowner will accept this as a normal business practice (which it is).

See the difference?

One method is under the table, and illegal. The other is normal business, and quite legal.