Buying used from an owner, what do we look for?!?!?

Check the DOT date on the tires. The last 4 digits following the "DOT" contain the week and year (WWYY). 7 years old would be the absolute max provided the tires look good otherwise. I don't think I would trust someone else's tires over 5 years old.

Sight down the sides looking for any bulges. Bulges would indicate that water has leaked into the side. Look very carefully for any signs of leakage. A small leak can cause a lot of damage.

Make sure everything works.