I have a store because it is vastly cheaper for me to list that way. I don't do auctions, so the store is the go.

I have no idea, because I can't be bothered looking, as to where my sales are generated. I use a store because it is economical and I don't really care where the sales are generated.

I suspect that the bulk of my sales (books) are from people doing searches on title and author. As I have few multiple sales, I don't think the buyers are searching my store for books and loading up their purchases to take advantage of postage discounts.