I'm a fan of getting most things used. I think with car seats, if you can trust the seller that it hasn't been in an accident, it's fine to buy used. But, I think that is a better deal with an infant seat that you only use for one year. A convertible seat will be used for many more years. I still have my six-year-old in the one I bought for her when she was one. It's a Britax Marathon and I think I spent less than $250 on it, but even if it was that much, that works out to about $40/year. I don't think that's very expensive when you look at it that way. The Britax is very safe, comfortable for the kid, easy to install for you, quality-made and durable, and has cute covers. But most important, Britaxes have an additional tether that most other seats do not. Or at least, they didn't when I was car seat shopping. I admit it has been awhile. An extra safety device on something that is meant to keep my child safe in a car (pretty much the most dangerous thing children are exposed to) makes it the clear winner in my book, and certainly worth $40 a year.
We now have two kids and four Britaxes. We bought two of them new and two of them from a friend. My older child is getting to the point where she could move out of her convertible seat and into a booster. I don't intend to move anyone to a backless booster as soon as it's legal. Rear-facing as long as possible, five-point harness for as long as possible, is how I roll. I'm not generally a paranoiac but I'm a little cuckoo about car seat safety because my mother was in killed in a car accident. It's kind of always on my mind how dangerous cars are. In my mind, this is not the place to skimp.