Make sure you understand the condition
Some sellers will mark items as refurbished when this is not the case. Items in this condition are typically restored to factory settings and may have new internal parts. However the body may have signs of damage. In many cases items are listed as pre-owned or certified which quite often means they are sold directly through the original manufacturer/retailer. Regardless of condition, make sure you fully understand what you are getting when buying used. Not only to ensure you know the condition and how the item will perform, but also to ensure you aren't overpaying for an item which is used, when it is listed as refurbished.
Again, with items which are second hand or pre owned, prices are lower than new but not all used items are the same. Those which are restored to manufacturer settings and sold as "certified pre owned," will run a higher price than those which are simply used and being sold by a private seller. Know what the price should be by visiting a few sites, viewing comparable rates and finding out what the average price is for items you wish to buy.
Just because items aren't new, doesn't mean they won't work for years and produce the same quality performance as brand new. However, you have to know what you are getting especially with electronics. Whether it is a set of speakers, guitars, a computer or any other item you have to know exactly what you are getting. So, before buying from any seller make sure you understand what different terms mean and what price you should be considering when buying a particular product/electronic item.