They are not Good they are an average inconsistent team there's no need for a great bullpen pitcher on a team that sucks(unless he's young and will be there long term) that's a waste especially at millers age.Trading miller should net them a good return he's the best lefty bullpen pitcher in baseball but asking for top 15 prospects should get you an equally young cost controlled player or prospect in return.Gallo is part of a deal that gets you a danny salazar,lucas giolito or a blake snell not 30 year old bullpen pitcher imo.
Like I said, he's 30, $9 mil a year for 2 more years after this. If they can't get something on that kind of level, they shouldn't bother putting him out there. If the best they can do is some high ceiling prospect still in A ball, they can net that in 2 years.
Drew Pomeranz pitched half a good season in the NL West and netted a top 35 prospect not even knowing if he'll make it in the AL East. Miller's worth more than Pomeranz. In a game where bullpen pitching is very volatile, Miller's had a track record of consistency (in the AL East mind you). That's worth something and it needs to be worth a lot to a team in major need of bullpen help.