Thus far, most people have only commented on the actual condition of the album. There are many other aspects to consider about the album itself, including the recording.
1) Beware of mono recordings. Some people do like mono recordings, but personally, I do not. Before stereo existed, albums never had "mono" or "stereo" printed on them, because all that existed was mono. Others may be able to comment on when stereo albums began to appear, but typically, albums released in the 60s and 70s will say "stereo" if they are stereo. Stereo was a big deal at that time since it was new, so it's likely to be printed somewhere on the album or album cover.
2) If you are shopping for classical albums, pay attention to the label/recording company. I am still learning which labels are the best, but ones that I always prefer are Deutsche Grammophon, RCA Red Seal, Angel, and others. Once you learn all the good labels, you can then proceed with caution when purchasing albums made by off brands.
3) Original pressings may often times sound better than the new "audiophile" pressings. You can always use google to look up an album's "code", which will then tell you when the pressing was from. I think the original pressings sound better many times because the master tape used to create the pressing was brand new. I jumped on the audiophile re-release bandwagon a short while back, but quickly realized they don't always sound great. Some of the newer pressings are even made from digital masters, which to me completely defeats the purpose.