another car seat

we are very very close to having to buy another car seat. with the one we have now Caitlin can fit but its quite snug. and forget about trying to fit her in when shes wearing a jacket.
so i went to BabiesRUs yesterday and talked to someone there. they said yep, we need to go to the next level and reccommended a convertible car seat like the ones found here
http://www.toysrus.com/search/index.jsp?kwCatId=&kw=convertible%20car%20seat&origkw=convertible%20car%20seat&f=Taxonomy/TRUS/2255957&sr=1
these will suppossedly work as rear and front facing and can go up to 100lbs so we dont have to buy another one anytime soon (though when we bought her carseat we have now they said it would be awhile till we needed another one cause it went up to 30lbs or 1yr of age, and yet 7 months later here we are because she is too big for it)

i just wish they were not so expensive. i see carseats on Craigslist for about half the cost of retail. i'm just not sure if its a good idea to buy a carseat used, because you never know the history if its been in an accident or whatever despite what people say. but then again i plan on trying to sell mine on Craigslist once we get a new one. so i'm just not sure.

if anyone has any reccommendations, feel free to share!