if a bottle is looked after correctly then it is rare for it to fail a re test, and the majority of people selling on here do look after their kit very well, however theres always that uncertainty. I like to think of it like buying a used car, if its fairly new, or has just passes a re test, then you know its in good shape and as long as you also look after it no reason it should fail next time round!
Although if you have the money to spend, then theres no reason not to buy new, and you have the peace of mind knowing exactly how its been looked after.