1. Permission must be granted - it can't be presumed. Buying addresses and sending direct mail is not permission - it's spam, and it's likely to be ignored. Customers don't want to be bought and sold and then marketed to.
2. Permission is selfish. People grant their permission only when they see that there's something in it for them. And you've got about two seconds to communicate that something.
3. Permission can be revoked as easily as it's granted. It can also deepen over time. The depth of permission depends on the quality of interaction between you and your customers.
4. Permission can't be transferred. It's a lot like dating. You can't give a friend the authority to go out on a date in your place.