There are three mods that I appreciate if I'm buying a used bass:
1) Strap locks
2) Hipshot Xtender
3) A proper setup
As far as value, though, #2 is the only one with any dollar value to me, and #3 only serves to make me more likely to buy it in the first place. Claims of a "setup" work the opposite on the internet. If an internet listing says the frets were just filed or similar, I'm less likely to buy that bass because I don't know if the person that did it was competent, and I can't play the bass to see.
Mods like a "better" bridge or tuners are worthless to me. I may change pickups or preamp on my own, but with a used bass, these are worthless to me in terms of additional value. If I don't know what the original electronics sounded like, I can't say that the "upgraded" electronics are an improvement.