You are correct, the terms “buyer” and “seller” apply regardless of the use of money.
I suppose the reason is that none of us live in a world of barter. Also, the phrase “let the buyer beware” can be meant as a slur against entrepreneurs and commercial activity. On whether or not people were happier in times of a less developed commercial economy and its primitive division of labor, take a look at Rothbard’s article:
https://mises.org/library/freedom-inequality-primitivism-and-division-labor
