The hard part for me is that the monetary value of many of my basses is less than what I deem worth getting rid of them for. For example, I have a Jazz bass (SX refin'd body, Fender USA hardware/pickups and Aerodyne Jazz neck), I don't play it all that much, but I love that goddamn neck so much. If I parted it out or sold it, I'd probably get $500 bucks, and down the line, if I want a Jazz bass with Aerodyne neck, it'll probably cost me more than $500 and a bunch of aggravation to replicate this one.
My situation is similar, but not really by choice. It's a matter of economic reality. I have owned many nice and vintage instruments in my time, sold most gear off when I became a father. I regret parting with only a very few of the things I've sold off.
I do not mean to sound like I'm whining...my family is better off than the vast majority of people on the planet, but we went from two decent incomes to one after my wife had a near-fatal car accident a couple years back. We have medical bills out the wazoo and help financially with my mother-in-law and stepson, so it's been a squeeze and a struggle for awhile now.
I often talk here about how I personally don't want an excess of gear or other stuff (and this is sincerely true) and that one doesn't have to spend a fortune these days to have a perfectly good, giggable instrument (and I absolutely believe this) but I don't begrudge anyone who has the means or desire to own three dozen basses that cost $4000 each. That's their business and I am honestly glad for them.
One of my GAS cures, then, is that I rarely have any leisure cash. Impulse buys are out of the question, even though it also helps that I don't have a big desire to own a lot of stuff.
What I've learned is that I should appreciate the instruments and other things I am fortunate to have, practice and continue trying to improve as a player instead of fawning over new gear, and I have become somewhat skilled at modding and upgrading inexpensive basses.
If you start with a bass or guitar that is comfortable to you, has a neck that's true, can intonate and hold tune, that's the bulk of the battle. I buy new wiring harnesses, jacks and (usually) pickups, and bridge, tuners and nut (as needed...I typically don't feel the need to automatically replace these, but it happens) and I end up with nice instruments that weren't a big investment, have the qualities I like, and I don't have to worry about them.
Of course, good new strings and a setup can also do wonders for a bass that's been sitting neglected at a pawn shop...