I opened an account with Etsy about four years ago to buy some cute buttons I found during an internet search. A couple of years later I opened my first shop. I kept my first account (I call it my buyer's account) for convenience. I want to keep accounts, receipts, payments, etc., separate. In my case, it had nothing to do with leaving honest feedback. I had that account for about two years before I opened my shop. I have had two or three bad experiences with items I have purchased, but the matter has always been resolved satisfactorily after I contacted the Shop Owner.
Through the years, I have purchased over 1,000 items from other Etsy Sellers. They probably do not know I am an Etsian. Whenever I need something, I search Etsy first, before going elsewhere.
Regarding Treasuries, I sold a high-price (to me) item precisely because somebody included it in a Treasury. I sent a convo to the curator and thanked her for my sale.
I have not made many Treasuries, but bought a couple of items from the Shops I checked out in the process of preparing a Treasury. Go figure.
So, bottom line, in my opinion, I think a lot of Etsians are buying from each other. I know I do.