Get two for less than the cost of one, and they are practically new...
Graco Highback Booster Seat with Side-Impact Protection. This seat is designed to grows with your child. It's a 2-in-1 booster seat that converts from a highback booster to a backless belt-positioning booster. It's easy to install and secure the seat. It has a hideaway storage compartment, height-adjustable head support, and a cup holder.
We have two so we could keep one in each car. I checked Target, Walmart, and other sites. Looks like you can buy one new for $85 to $110, depending on timing/sales. This was a replacement set we bought and rarely used in a 2nd car. Our youngest child outgrew the need shortly after purchase. They have been sitting unused and are too nice to just give away. You have can BOTH seats for $50.