Must trim paint (for baseboards) be gloss or semi-gloss, or can it be flat?
Primer or primer/sealer is only necessary if the surface has not been previously painted (the question does not state whether or not this is the case) or for certain specific situations with "problem" surfaces.

Trim (like door/window casing, baseboards, etc.) are usually treated with a coating that has some sheen (gloss, semi-gloss) to facilitate easy cleaning and for resistance to scuffs and marks. Trim boards exist to protect walls and surfaces from scuffs, marks, and impact damage in vulnerable areas, hence the likelihood of receiving such and the necessity for more durable finishes.