If you pay attention to the news on even a glancing level, you'll have heard of the scams that have plagued online public markets such as eBay and others like it. Unfortunately only a few bad apples have turned an otherwise great service to what many people think is a huge risk. While there are a few risks associated with shopping online for used items, it doesn't have to be that risky. Sites like ebay now provide a comprehensive ratings system for customers to use. In general you should look for a seller with a high customer approval rating.
When it comes to online markets, the most important things to look at are a sellers approval rating and how many people they have sold to. If you look through various sellers you'll notice a percentage next to their username. This represents how many people have been satisfied with their order. Then there will another number next to the percentage, this is how many items they have sold. Online markets also create forums in which individuals can rate their experience with their seller. In general it is safe to go with a seller with a high approval rating and has sold to many different customers.