You know, I've struggled with this same question. My shop is small and new, but has so far been successful, considering the tiny niche market my items appeal to. The conclusion I have come to is that I have no way to police people who are selling items made from my patterns. I just don't see how I could possibly have any control over what people do with my product once its in their hands. I also feel like the people who will use my patterns to make items to sell are not going to prevent others from buying from me. Assuming sellers respect my wishes for credit and links, it should direct more traffic to my shop. That said, the following policy is my happy medium:
A few words for Etsy sellers and professional makers of doll clothes...
I am a work at home mom, designing these clothes, drafting the patterns, then writing and photographing the tutorials all while my two year old sleeps (or plays quietly, HA!). I realize that it is impossible to ask or expect my customers not to sell the doll clothes they make from my patterns, but please understand that it is an extraordinary amount of work to produce these documents. I hope that my customers, who are designers and work-at-home mothers like me will respect the following wishes if they will be selling clothes made from my patterns:
**Please give credit where credit is due- Include in your item description, "This dress was made from a DalaiMama Pattern" and a direct link back to my shop listing for the pattern you purchased. This also applies to any promotion via blog, Facebook, Etsy, personal websites, bigcartel, etc.
**Please do not mass produce my designs or offer "custom orders" made from my patterns. If a customer messages you privately to create a custom order using one of my patterns, that's one thing. But if your shop has "custom slot" type listings, I ask that you please do not offer my designs for sale in this way.
I'm not working on the same scale as you are just yet (congrats on selling a pattern to one of the "big 3") but this is my take on the whole situation. I'll be staying tuned to see what others have to say about this.