Some interesting thinking there, John. I have a dabble occasionally into buying and selling the odd bass, but I'm being quite cautious at the moment, what with the market being rather flat!
Mainly I buy them to play. I look at each one as a tool of my trade. If I can't justify it being around, it has to go. For example, I bought a black Hofner Contemporary series bass last summer which I've now traded in against my Marcus Miller. The Hofner was bought for a supposed 60's set which worked about twice. I'm more likely to use my Precision for such jobs, so that's infinitely more useful as an all rounder.
The next one in question is my KingBass which sits prettily in it's case at the moment. My "signature" bass since 1990 has been the Status SII (I'll have three of those operational soon) so the KingBass, nice though it is, is looking pretty surplus to requirements.
KingBass Artist, anyone?