In Australian English, acreage brings to mind a large piece of rural land, large enough for production farming and much bigger than a house. Although etymology isn't always in sync with the contemporary use of particular words, in the case of acreage, the etymology of acre fits:
"[O]riginally 'open country, untenanted land, forest'; ... then, with advance in the agricultural state, pasture land, tilled land, an enclosed or defined piece of land" [OED]. In English at first without reference to dimension; in late Old English the amount of land a yoke of oxen could plow in a day, afterward defined by statute 13c. and later as a piece 40 poles by 4, or an equivalent shape [OED cites 5 Edw. I, 31 Edw. III, 24 Hen. VIII]. The older sense is retained in God's acre "churchyard." Adopted early in Old French and Medieval Latin, hence the Modern English spelling, which by normal development would be *aker (compare baker from Old English bæcere).
There's no transactional connotation to the term that would imply that the land is for sale, unless the context specifies it.
Here's an example of the term used in an insurance ad:
The typical Small Farm policyholder will be someone who has a small acreage, with enough space to keep a few animals, pony for the children or grow vegetables for their own use and maybe selling some at the local weekend market.
The description is an example of the use of the term acreage in a context that doesn't necessarily have anything to do with buying or selling the property. Now, there are instances of 'acreage for sale', but that doesn't intrinsically tie acreage to sales. (Compare this with 'house for sale'.)
You also ask:
Would it be pragmatic to use acreage to refer to any plot of land that, theoretically, could be used for agricultural purposes?
Based on the above, such usage would likely be fine, unless the context strongly suggests otherwise.