I've had my store for 18 months and to date I have 5 subscribers - a couple who (as far as I know) have never bought from me and a couple who have. It was really only after I started specialising in a fairly specific category area that people seemed to subscribe, and I don't have any links in prominent places, or even really give people much of a reason to subscribe. (I have emails featuring new products set up to go out monthly, with the link in my side-bar menu of the store).

To encourage subscribers, I think you probably have to be more active about it, maybe put a promo box above the listings on the store pages, set up the emails so that people can subscribe to categories of specific interest (I have 3, so that people can subscribe to everything, or an individual category of item).

To be honest, I don't think they encourage too many sales - I subscribed to a few stores once myself, and while I opened the first couple of emails, I ended up deleting them all without reading and if I add a seller to favourites these days, I make sure I'm not subscribed to any emails. These days, if I want to keep up to date with a seller (on eBay or not), I just find them (if I can) on Facebook, as updates are generally in real time, and being on a more social level allows for interaction in a range of ways (I'll usually unsubscribe from most feeds if all they ever do is post links to items for sale).