If your talking about friends for Sale basically someone who has added the application and by the money they receive for adding the application and logging in (fake money obviously) to buy you, once they have done that you then become thier pet. It doesn't matter if you have the application or not they can still do it. If you want to join in go to where it tells you you are someones pet then click on friends for sale or buy and sell and it should take you to the relevant page
What does these things mean on facebook.com being someones pet, buying someone for money and the points?
There are several pet applications on Facebook, (Owned, Human Pets, and Friends for Sale, etc.) but they all basically work the same. They are a type of game application, except you earn "money" instead of points. On most of the applications, you have a mini profile. Most of these applictions have poking and messaging features. You can browse the application, and "buy" people with fake money that you earn from using the application. People are basically buying your picture, which is displayed on their page in the application. When people buy you, your value increases. You also earn "money" on most of the applications when you are sold for a higher price than you were bought.