While there are some total bargains to be had, buying second-hand can be risky, especially if you don't know exactly what to look for. First and foremost you can say goodbye to any manufacturer guarantees and warranties, and it's almost impossible to prove if any previous maintenance goals have been hit correctly and on time. For some tell-tale signs of things to avoid, take a look at our article on 6 signs you need a new mountain bike.
Our advice would be to visit a tried-and-trusted bike shop or legit, customer-service-focused online retailer.