I have always wondered about this. maybe someone k

I have always wondered about this. maybe someone knows correct answer. lets say " mm abcd" has 10,000 for sale at .20, so lets say i'm trying to take him out but i am trading thru ameritrade. when i put in my order could not ameritrade just sell me shares that they have on hand if they choose thus would not show up as abcd selling shares. hope i'm making sense.