I can select seats on an MH itinerary issued by my TA. The cost is the same.
However I do not know if this is a TA-specific setting, as you cannot select seats on QF flights issued by some online travel agents.
As an aside, I’m personally a bit suspicious of finding tickets that are that much cheaper on an online travel agent vs direct from the airline. Just make sure you’re getting the same inclusions on your ticket.
