going to buy a used pool today what should i look for?

You'll be awful lucky not to need a new liner to replace the one that is 4 years old. Age is not the problem. It's the fact that is has been removed and dried out, making it brittle.
 
Look at the bottom of the support posts...pull away any backfill dirt if necessary...and check for rust and corrosion. When I was looking at used pools, nearly all of them had SOME posts that were going to need to be replaced, and with an older pool (especially if you can't determine the manufacturer) that can be a problem.
 
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