Word for "item that can be bought or sold"
Solution 1:
I'm no expert and I don't feel entitled to answering a question. But since no one has come up with these words, I propose:
property non-property
What concerns Native Americans about land is whether they own the land they live and use.
So it goes, Native Americans did not consider land to be (a) property.
Like it or not, if it's their idea that land is not synonymous with property, then I think that property can be rightly used in OP's sentence.