I really advise you not to buy from a garage sale or resale shop. Buying a used car seat is a big "NO" because you don't know the history of the seat and if it has been in an accident. Even if it looks ok to the naked eye, it still may have damage to it and not work properly in a crash. In some states, it is against the law to sell used car seats, and it should be that way in all states because it is unsafe. Plus, car seats do expire. Usually 5-7 years after they are manufactured, and a lot of used seats should be thrown away because they are expired.
I would advise that you buy the Evenflo Triumph Advance at about $130 (there are coupons out there for BRU, Target, etc) or the Fisher Price Safe Voyage car seat (online). The Fisher Price is made by Britax and mirrors the Marathon but is only $130. So you get Britax trusted name & quality for half the price. They will both last you for at least a couple of years, if not more. Check out albeebaby.com, they always have great deals.
One more thing, there are huge car seat sales this week on Britax and other seats due to the car seat show that was last weekend. It is the only time of the year that many retailers put Britax on sale. Just in case you do decide to go that route.