You listen to offers for anyone, "make me an offer I can't refuse."
I think there may be some players who aren't good in the lockerroom or who aren't coachable that they might sell high before the grapevine makes those flaws common knowledge. Not suggesting that applies to those three, just that at times there are issues that don't surface until later.
However, when you're rebuilding, there's no reason to make a trade just to make a trade when it involves players with upside, in effect you're just trading one gamble for another. Trading Herrera is unlikely simply because you'd have to get an OF back, Quinn and Cozens are unknowns, and Moniak, Ortiz and Randolph are a long time coming.
One reason I don't expect much to happen this winter is the performance at Lehigh next year will answer a lot of questions, and make it easier for Klentak to figure out who to include in a "blockbuster" trade. If Joseph and Hoskins both hit well, one will be included in a trade package, if Altherr rebounds, Cozens steps up, etc. If the young pitchesr improve, there will be an excess arm or two that can be included in a package. It'll be much easier next winter to project the 2018 lineup, know who is blocked, and where they need an upgrade.