I would be a little skeptical- it would be hard to know for certain that someone didn't try it out and repackage it. I think you also need to be careful of how they're shipped- if it's tossed around a lot or dropped, it could affect the integrity- many car seats that are in even minor accidents are supposed to be replaced.
If you're searching for a deal, keep an eye out for online discount web sites (i.e. Zulilly) and check Amazon. If you have time for the next BRU trade-in sale, you can trade any crummy piece of gear (i.e. old hand me down or something junky from a garage sale) and get 25% off a new item. Target also has some amazing clearance deals if you can catch them- when I was pregnant, they had Graco Snug Ride + the coordinating snap n go for $75, but were all sold out by the time I got there. Car seats and cribs were really things where I wanted some peace of mind.