We all love a great deal, but it's important to remember that when venturing into the world of third-party retailers — the kinds that sell through Amazon, Buy.com, or eBay — the bargain hunting gets a bit riskier; while you might find a lower price, you might also face myriad headaches, including fuzzy buyer protection and shoddy customer service. Here are seven questions to consider before you buy from a marketplace merchant that will help spare you from unnecessary grief.