So long as you do just engrave it and not cut straight through the whole thing, I see no reason why any harm would come to it.
To be honest so long as you dont cut through the runners and stuff, cutting all the way through shouldnt do anything bad so long as it was over the outer barrel part rather than the blow back unit etc
to be honest i doubt ill ever consider selling it! & Hopefully that wont happen (fckin hope it doesnt anyway!) but ill find out monday, wish me luck! haha