To emphasize what Doc just posted, I collect lots of old tapes. If I'm buying from eBay, I only consider ones that have a picture of the reel available. The 2 things I look for in those photos is if the end of the tape is secured (rather than with a couple of wraps loose) and if the tape has a nice pack (shinny and regular). If those two conditions aren't met, I move on. When a tape is wound, it's under tension (whatever the tape tension is set at on the machine doesn't just go away once it's on the reel) and if the tape isn't wrapped evenly on the previously wound layer of tape on the reel, that tension will pull the tape toward the hub and over time, will curl that edge inward. On 1/4 track it's immediately obvious when this has occurred. Even if the seller has wound the tape correctly so it looks nice for the photo, if the tape backing has curled, the left channel of that side of the tape will not make consistent contact with the head and at the very least, the left channel output will be reduced (the playback machines tape tension will be fine on the un-curled portion of the tape but the curl is actually a stretching of the backing, increasing the length of the curled portion and reducing tape>head contact and decreasing the output).
I always store my 1/2 tracks in what is called library wind. This is tails out wound at playback speed. It's kind of amazing what kinds of abuse some old tapes can take when stored in library wind and equally amazing how little it takes to ruin a tape which isn't.