Think "fresh" when renting your list. One more bit of information you'll want to find out is the deliverability rate of the list. Granted, no list is 100% deliverable mailing list due to the fact people are always moving and changing addresses. This is how you get undeliverable pieces also known as "nixies". You should at least be able to find out how often the list is cleaned of outdated addresses and updated with "fresh" names.
When placing an order with a list broker, the minimum order is usually between 2,000 to 5,000 names. Most reputable list brokers don't mailing list sell orders less than 1,000 names, and even that is rare. Of course, don't be afraid to ask for a smaller number. All they can say is no. Finally, make sure to take your time to choose a list that fits what you are selling. Don't try to cut corners when renting a list.
Do your homework and make sure the list matches up with what you're promoting. This part of marketing is costly so you want to get it right and get the best results possible. If you're following the advice of people, who are already making money mailing list online, you're probably building a list. And the easiest way to get started is to offer something for free. The advantage is faster list building.