I have bought a used blower and had no problems.  It depends on the type
of blower.  What brand/model and how many miles are on the unit you
looking at buying?

Does it had the right hardware to install on your car? Is there damage
to the impeller, rotor or screws?

I just had my P-1SC apart and it has very little backlash on the gears
and I could not feel any end play on the bearings. It cost me $15.00 for
the drive pulley seal.

This was after 30K miles, a 3" pulley making 10lbs. of boost and mostly
street driven.

Absolutly, they are just air pumps.
Check for all the small parts to see if they are there.
The MFGR can overhaul it too.
I got mine that way, in East Texas, came off a wrecked race car, It worked
fine for 10 years.

   I almost did, I just couldn't afford it at the time and they were
asking a wee bit too much. I'd look for abuse/visible wear, but other
than that make sure it fits *your* application with the included hardware.