For that price, I'm pretty sure you could get a new 54" 12x20 pool from poolsupplies.com.

I just bought a 18x33 oval from them, the atlantic pools j5000/vantage, and spent less than $3,000 out the door, including their deluxe accessories package, a pump/filter package of my choice, cove molding, wall padding, and a ton of other stuff...

By the time you pay for a liner, you'd have spent nearly as much on a used small pool as I spent on mine.

I'd run like heck from this deal.