Generally, its fine to buy a 2nd hand textbook, although, if you can, try and get the right edition...
for example, from 2001-2004, my uni used Knox et al. (second edition) for one of the first year biology streams. However, this year, they are using third edition as their prescribed text, in which (apparently) the fungi section has essentially been re-written....so, having bought a copy of 2e, one may be incorrect whilst studying for fungi.
however, I had a old edition of Gleitman et al. for psych, and I found it (the, you know, maybe, twice, I actually looked in it) to be very similar/identical to the latest edition, only thing was, I occasionally had to find stuff myself coz pages had changed.