Its a difficult to read proposal (you should really try some paragraphs).
What he's saying isn't that he wants vendors to sell items, but that he wants a system where he can register an order and anyone can go fill it.
There would need to be some sort of reservation system, otherwise you'd go make some obscure, little needed item, and meanwhile 3 other guys did the same, and only 1 of you gets the payoff, while the other 3 get nothing. So maybe you can "reserve" the order for some period of time while you go make them.
Anyway, I think the current system works fine:
1. You can make most subcombines you need, even cross craft, you just can't make them as well or as easily as that class.
2. You can go buy the items you need from a broker.
3. You can look for "LFW" crafters of the appropriate type, and commission the items you want from someone of the appropriate artisan class and level.
I kind of like your idea, don't get me wrong, but it would be difficult to implement properly. And I'm not entirely sure its needed.