everyone's tradeable so i'll answer the question as if it's asking who i would least want to trade away.
for personal reasons, Avery Bradley.
but for business reasons, Sully. severely underrated by non-Celtics fans imo, and rebounding is a talent and he's got it in spades.
Meanwhile, Rondo is valuable enough that he'd be a very good trade chip. I feel like i've said it a bunch of times but if we could land Wiggins (via our own pick) and Exum (via Rondo trade), i'd be thrilled (granted, we probably still won't be winning much games in 2015. the NBA isn't a rookie's league).