I often think that used instruments are more difficult to get the action right than a new one, as you don't really know where the previous owner kept it, how he kept it tuned, what strings it had, how it was stored, just a lot of its history you just can't know.
I'd guess that as it's buzzing from the middle of the neck to the nut, that there's too much truss rod (tightened up to where the middle of the neck is bowing up relative to the ends of the fingerboard) and the nut is too low. I'd guess the previous owner did the usual 'adjustment' of raising the bridge to kill a little rattle and never touched the truss rods, or tightened them up way too much, which isn't hard.
If you're new to these adjustments, remember that a seemingly small adjustment can make a big change in feel. We deal in quarter-turns here or less.
Pick up your bass by the body, hold it out parallel to the floor, and 'sight' down the neck with a good light falling on the fingerboard. Hold it by the body only, do not support the neck or head anywhere. Regardless of the shape it's in, you will see shadows of the strings falling on thefingerboard beneath the strings.
The strings are a basically perfect straight-edge: The shadows will reveal the shape of the setup. Their shape will tell you what the setup is. When mine is right, the shadows are straight with the strings with just a bit of a very slight dip in the middle of the fingerboard, and a slight hump are the top of the fingerboard.
Do NOT do this by laying the axe on a table and picking it up by the head, that's the WRONG way to do this. We are working in clearances of thousandths of an inch here, and virtually all guitars are heavy enough to add a little neck relief doing it this way, so don't you do it.
This is why I never use a neck-hanging stand: I store them in the case, body end down. The stands I use, all the weight is on the body end.
I'd guess you are going to see the stings' shadow arcing up towards the middle, though I could certainly be wrong, hard to guess without holding it in my hands.
I tend to think that these axes could be affected by weather so easily like an old man's bones are just not usually the case. They are planks. There will be the occasional problem child wood-wise, but it's pretty rare. I always use the same strings, keep them in their cases in a typical home, and I rarely have to even re-adjust them, until I get the bug up my ass to try new strings . . . . Now if you leave them in a hot trunk all day, or a cold trunk overnight as you were too tired to bring them in when you got home from the gig, you're on your own.
I would first put on new strings, the kind you intend to run on this from now on, then dive in. VERY light or heavy guage strings may complicate this somewhat. There is no universal or perfect setup: Only what works best for you. We're all different in our technique. The beauty of these axes are their adjustabilty that enables you to easily find YOUR perfect setup. None of this we learned overnight, I chased it for several months at first, until I 'got it'. Now I'll never have to take it to a 'professional' again, only to have it go South on me after I spent that money !
Best of Luck,
J o e y
