Yes, Marx is referring to the buying and selling of land.

Theoretically he's correct; the buying and selling of land results from some of the highest degrees of development of the money-form. Generally it is the last thing to be commodified because the idea of 'owning' it in the liberal sense (where things are the possession of one person and that one person has sole authority over its use, sale, etc) is so completely absurd.

Generally land is considered to either own the people or is considered to be some form of collective property (e.g. of a peasant village or family among whom the land is redistributed periodically) or the property of god on whose behalf the king and his nobles manages things and hence none of them have unlimited rights over it.

Historically, Marx is a bit off. The development of bourgeois society in Europe began much earlier than he thought, particularly in England (where, by 1200, this sorta land ownership was being practiced in some areas, by 1400 it was widespread and a large class of agricultural wage-labourers had arisen). I haven't done enough investigation myself yet, but I get the impression from what I have read that Rome's property-laws were closer to bourgeois absolute property, which makes sense because it too had a very developed money-form (as Marx alludes to with his Apocalypse quote).

In terms of his point about nomadic peoples, I think he's correct, but only in cases where the nomadic peoples exist alongside settled city people. Marx's big issue (and the thing that makes anthropologists think they're proved him wrong on e.g. origin of money) is that his historical research is restricted to the last ~3000 years, and doesn't have much in the way of archaeology or history to go on. So the examples of nomadic peoples Marx has at hand are 1. pastoral peoples between egypt and mesopotamia 2. steppe/pastoral nomads around europe and maybe china 3. biased accounts of fur-trading natives in north america. In all cases these sorta nomadic people do seem to create money systems, but without impulse from the cities this doesn't happen. Trade still occurs, but it becomes ritualised, focused on use-values, turns into gift-economies, etc.

Could it be tied to alienation? That land could be alienated and turned into a commodity is not self evident, and because land is literally fundamental to sedentary societies by the time anyone would have the idea it has probably been enmeshed in the social existence of the entire community and thus not easy to take private control of. A desperate laborer can alienate their own labor unilaterally, but a desperate farmer can't alienate land that is collectively worked by a village.

You'd need to suddenly appropriate land, like Henry VIII did to the catholic church, for there to be an alienated chunk of it that has no social resistance and can be disposed of in a simple transaction between owners of commodities.

I saw this part in chapter 3, and I feel like it's relevant to what you quoted.

Hence an object may have a price without having value. The price in that case is imaginary, like certain quantities in mathematics. On the other hand, the imaginary price-form may sometimes conceal either a direct or indirect real value-relation; for instance, the price of uncultivated land, which is without value, because no human labour has been incorporated in it.

mainly it just kind of seems like Marx is building up to how private property came to be?