Buying from them is probably no riskier than buying from a commercial store with a Web site, since they're usually authorized agents of the major brand manufacturers. Price is probably going to be a big consideration for a you, but it shouldn't be your only consideration. To fill the void, you will take up golf, a hobby even more expensive and frustrating. This is not entirely true. This is called ground balancing.
Some detectors come from the factory with the ground balance set to a pre-set level.
Others allow you to do it yourself to adjust for various conditions, depending on how much iron shows up naturally in the ground. If you're going to hunt for coins and relics on land on beaches, in parks and around old houses, an automatic is fine. There are also detectors made specifically for those who plan to dive, gold prospect or search wet sand areas of salt water beaches, or beaches with black sand. On salt water beaches, the interaction of salt in the water makes wet sand conductive. In other words, the wet sand will take on the conductive properties of metal without being naturally metallic.