It would seem a simple thing to touch up brassed pens and pen trim with a bit of electroplating. The reality is otherwise, and in large part because the trim to be restored was typically not originally plated at all, but rather gold filled that is, a laminate of base metal and gold, mechanically joined (as such, "replating" is a bit of a misnomer). As a result, a brassed spot will have a raised ridge and often an area of porosity where the top layer of the gold laminate is worn away. If this is not carefully polished smooth beforehand, a telltale line will remain visible after plating. Simple buffing will not be enough, and if anything is likely to make the line more pronounced. It may also be necessary to plate on an intermediary layer of copper to provide a sufficiently even surface. Few platers are interested in putting in the requisite time and effort.
The other problem is the thickness and quality of the coating. Give a pen to a plater, and nine times out of ten you will get a microscopically thin deposit of high karat gold a layer so thin that its transparency will reveal the worn area below, and so soft that it will wear off before long with only light handling. Thicker and more durable plating is possible, but it is not easy to find a plater both willing and able. Even then, in nearly all cases you will end up spending more to have your brassed pen properly plated than it would have cost to have bought an unbrassed pen in the first place.