[webmaven]

> At what point does this sort of thing become legally questionable?

Only at the point where the roll-up creates a dominant player (or several, ie. a monopoly, duopoly, or oligopoly[0]) and antitrust rules kick in to prevent further consolidation or abuse (of various sorts) of the dominant position(s).

[0] In theory, antitrust also protects from abuse of monopsonies etc., but abuse of a dominant buyer (eg. Walmart) is harder to accomplish and maintain, harder to prove harm, and harder to regulate.