Depends who you buy from.
It's a risk just like buying anything online, but if you stick to people with high feedback percentage (my personal rule of thumb is >98%) and who have done multiple transactions (I tend to stay away from people who have less than 100 feedback, unless they're offering a one-of-a-kind item that is worth the gamble for me). But it's not a store like Amazon is, unless you're referring to the "buy used" section of Amazon, which I personally hate.