Well, it is an old car. You are betting on its reputaion.

If it was a Taurus,( almost same size, same engine power.)-- it would be worth problably $3000 (my wild guess). So you are paying alot of extra money for its reputation.
If the car is in the north, you will face rust issue...brake rust, body rust...that just comes with age of the car.
If you check online, Avalon has lots lots of happy onwers. Everyone seems to be loving it...