Thanks everyone. I had also heard that registered users were around 800,000, with the largest percentage being buyers. The 12,500 for shops seems low since Etsy is worldwide. The May Etsy Weather Report states that 688,365 new members had joined, up 935 from April.
I suspect the total number of registered users is much, much higher. Oh well, I'll just have to throw in some generalized language on Etsy background, sellers and buyers.
I can say one thing, doing this business plan with a business counselor is really making me focus on what I should be doing.