Used crib mattress

A friend of mine got her 4 year old daughter a big girl bed and is seling her $300 naturpedic mattress. She offered it to me for $90 before she sells it on Craigslist. It's in perfect like new condition (I saw it in person already) and she had a waterproof protective pad on it. Should I buy it from her or buy a new one? I was thinking of Colgate eco iii or the little castle one. But I also know that naturpedic is all organic and for $90 that's an amazing deal.