Unless your buying brand new it's always a bit of a risk.Myself i've only just got rid of my 1 series bmw and traded it in for a used 4x4 which low and behold already has problems.I've a months warranty so,it's gone back to the garage to be fixed(which they've said they will do).
As for the sellers,i know they are somewhat eager for your custom,it's they're job at the end of the day so i never really see it as dirty or dishonest.
At the the end of the day,make sure you know what you want before you leave the house,don't worry about the spiel the salesman will give you,if it does not fit your criteria,either spec or financially,then just leave it.If you think it's right for you then give it a go,at the end of the day it you don't like it after buying then you can always trade it in and try something else.