I'm a fan of Mountcastle, but you consider trading anyone if the offer is right. But, I still think he's undervalued. He's a plus defender at 1B and slightly better than a league average hitter. Mayo has not proven he can play on the infield grass. In fact, his performance argues against that. Personally, I think Basallo will be the replacement at 1B in the long term.
And, if the Orioles are going to trade a veteran fielder during the off season, I think it's more likely to be Mullins than Mountcastle.