Most business practices involve selling something for more than you paid for it. In order to do that, you need to let people know that you have the said something. Generally, if you tell enough people, no matter how much money you ask, you will find someone willing to pay that amount for said something. Here in lies the trick.
How do you tell enough people? Marketing and advertising. You can buy space in a publication. You can paint a sign and mount it where it will be seen. You can simply walk up to people and tell them. You can get their address and send them a letter. Or e-mail address and send them an e letter.
For people who are involved in selling something for more than they pay, getting the message out is one of the most important activities they can perform next to actually taking money and handing over the goods. So they collect names and build a contact list. Next to their inventory, their contact list is their most valuable asset.
Nowadays, people are blitzed by other people who want to sell them stuff. Once a marketer gets your contact information, look out! You will get contacted. An easy way for marketing people to get your contact information is to offer a benefit for providing your valuable information. You sell your contact information for something. The least costly to the marketer is a free draw. Another option is to provide something of little monetary value. Say a laminated card. It is ok to provide your information, just know, nothing is ever "free" and as long as you are happy to pay the price, go ahead and spend.