Building a newsletter to keep in touch with the people who visit your merchant affiliate site is a great idea. Getting people to think of you and return to your site will be a great boost to your merchant account affiliate program.
Create a sign-up form on your web site so that it's easy for people to register for your newsletter (most people recommend using a double-opt in sign up process so you don't get accused of sending spam).
In the newsletter, encourage your subscribers to forward the newsletter to a friend. If you participate in online forums, add a line about your newsletter with a link to the sign up page in your signature. Put your newsletter archives on your site so people can see what they're signing up for and search engine spiders can index the contents.