Who are market makers on Ripple, and are they considered money transmitters requiring licenses?

This is a tough one. If I am a market marker, I am bringing in trade for the XRP on some currency pair. Because I own the pair, I may not be acting as an intermediary, so I might get away with being a money transmitter, but as soon as I breach the various limits (set up by various US States as to what is the limit, over which you become a money transmitter).

IANAL disclosure. However, in most cases, anyone that becomes a market maker is most likely going to fall into the gambit of being a money transmitter, as the trade they are making the market for, does not belong to them, i.e. they are neither the owner of the transaction nor the beneficiary of the transaction - hence they act as an intermediary, thus a market maker.