Buying a used car seat means you are trusting the seller to give you information honestly. Can you trust a stranger on the other end of eBay to tell you whether or not that car seat was in a crash? Do you know the person at the yard sale well enough to know that they are car-seat savvy and have maintained the car seat according to manufacturer's instructions? When you trust a stranger to give you that kind of information honestly, you are essentially trusting them with your child's life, so it's important to make that choice wisely. My suggestion is to budget for a new car seat and cut costs elsewhere if need be.