The Watch Seller link appears on each listing in the section of the sidebar which gives details about the seller.
Pressing this link puts the seller on your Seller Watchlist. Go to your account and click on Watched Sellers to see it. You'll see a quick overview of the current open listings by all sellers you're watching.
You'll also get a daily email containing the details of all new listings by sellers on your Seller Watchlist. This is a great way to stay ahead of listings you'd like to bid on.