> 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.
Currency (spot) markets are not typically public. They are usually OTC deals between 1 counterparty and a specific bank. Even on multi dealer platforms you don’t see the same prices other people trading on those platforms see.
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.