It is not possible to buy your own words. All the 3 word addresses are randomly allocated and completely fixed, meaning that they can be used in offline, which would not be possible if people were able to make changes all the time.
We have considered, and tested, a system where people could buy words of their own choosing. However, such name changes would not be available to offline users, undermining the idea of what3words as a universal system that never changes.
If you don't like the 3 word address for a location that is important to you, then this article has some further information and ideas.