Jon's business is his business but as we are discussing it here I frankly state I don't see where would the problem be.
If I sell oranges and orange juice, and somebody else buys my oranges to make his own orange juice to sell, that's fine for me.
If Ikea sells "mount it yourself" furniture kits, and somebody buys them and mounts them for the final client, so be it. The fact that the Ikea instructions are handed over to the final client is not relevant. The Goodman mark might actually have been removed for fear of infringing on some trade norm.
In any case the other agent performed an intermediate step for the client, as directed by Jon's instructions, and handed the resulting good to the final client, with Jon's instruction. Doctoring the author of the instruction is the only debatable part, but a case could be made that correctly the other person omitted Jon Goodman trade mark for fear of being accusing of stealing its name*.
I see, on the contrary, as an unfair business practice to restrict sale to a client who is going to perform a productive step from semifinished good to finished good. We should ask how would we feel if Ikea didn't sell furniture to us only because we mount it instead of the final client. And as a final client I ask if it is legal that the sale of a semifinished product is negated to a competitor because the seller of the semifinished product also sells the finished product. Can the seller select his buyers? (in many jurisdictions he cannot. Don't know about the US. But if we talk ethics, well, in my ethic jurisdiction nobody can negate business to anybody who agrees to his terms).
Jon was in two businesses of this "cottage model" industry: the business of selling a certain semifinished product, and the business of selling a certain finished product.
If really his goal in business life were to squeeze out somebody who buys his semifinished product, his business answer would have been to raise the price of the unfinished product and lower the price of the finished product. He might have discovered that his unwanted client would have gone somewhere else and that he lost business for no reason.
From a strictly business point of view I think Jon has no sound complaint here.
Selling only the finished product, and selling through his own site, are decisions that he could have made regardless of any other consideration, and which might turn out to be very valid ones, but if I am prepared to sell a semifinished product to a client (giving up on the added value of the cutting work, which I don't perform), what's the problem for me if somebody else, instead of the final client, makes the cutting work for a fee?
PS Sorry for late replies, I'm leaving tomorrow for a week.
* The objection "he could have written his own instruction himself" doesn't hold in my opinion as the intermediate performer performs, so to speak on behalf of the final client, a productive step as intended and described by the kit producer, and correctly hands over the instruction of the kit producer to the final client for the performance of the final productive step as intended and prescribed by the kit producer.