1. As contracts go, the fact that the Europeans had canons and muskets renders such contract voidable due to duress (cf. if a mugger points his gun at you and asks for a gift of your wallet). The management in Europe was strongly against stealing from the Indians, but that had minimal impact on the people on the ground in the New World. The Indians knew they lacked the ability to work iron or make gunpowder, and took whatever they could.
2. Most Indians did not have a European concept of “title” but rather a non-exclusive right to use the land. Buying access didn’t mean you owned the place. They weren’t “selling Manhattan” – at most they were granting an easement to build a trading post.