Just for mail order companies

Years ago I use have a small mail order business. Most of my customers came from requests for my catalogs. Now as I look back I could have profited from a direct mailing list. There are companies that sell or rent mailing lists. They are known as a list broker. I am sure they do the same thing (have lists) for email addresses as well as direct mailing lists but I have never looked into that. I think the internet as a whole has made it much easier for individuals to start and run their own business with ease.