Why did White Settlers believe they could buy land from American Indians?

First of all, the proper term is Native Americans, not American Indians.

Also, Native Americans did not believe in land ownership. White settlers took advantage of this. They did not see Natives as their equals, but more as animals. They felt that they were entitled to this land because it was not currently "owned" and they were white. So, the short answer is racism.

White settlers believed that they could buy land from American Indians because they believed that they could use the land more profitably than the Indians. Although they tried to buy, it raised a lot of conflict between the European settlers and the Indians.