no, not usually. There's nothing wrong with buying a used one, as long as you can get a history from the privious owner, which can't happen at a thrift store. What you need to worry about is whether or not it's been in an accident because that can compromise it's integrity. Even if LOOKS fine you need to be worried about stress that's been put on the plastic and possible hidden cracking. Also, there is the issue of recalls. If you see one at a thrift store and you decide to buy it, have them hold it, go home and look up the model on the internet. Find out if it's been recalled and/or if there are online manuals you can access for proper use and installation.
Long story short, it may be cheaper to buy one used but just BE CAREFUL!! Your baby's safety is worth more than you'll save. Buy everything else used :) ((Also, check Craigslist - there is a TON of stuff on there for cheap!))