I am pretty confident in saying that I'll sell any instrument if it stops floating my boat. My tastes change.... er become more refined. I plan on letting the instrument I am playing keeping up with me. I guess I am maybe not as sentimental as some others about their instruments. I find that odd as I know that I don't flip instruments nearly as fast as others on here who have instruments they'll hold on to for life.
I do try and hold on to an instrument for a while after I stop using it. I know that sometimes a little time away puts the bass in a new light. If that light is positive, I'll keep the bass and start it up in rotation again. If it isn't, it is out the door with it. My current bass in cold storage is my Streamer LX 6 Wideneck. It was my #1 go to gigger for around 8 years. I've done literally thousands of gigs with it. Now after all this time it isn't making me as happy as it once did. I am going to give it a while and see if the love affair rekindles. If it does, great. If it doesn't, out the door with it.
As close as I will probably ever get to the "I'll never sell it" camp, is my old shortscale acoustic fretless. That bass is a collosal piece of crap. I'd sell it in a heartbeat if I could find a sucker to give me a bill for it. That'll never happen so it stays with me I guess.