A movie marked "Used" was acquired by the retailer by trade or purchase at a discount. The movie could have been owned by one or several people. It may have last been stored in a pristine media cabinet or in a drafty garage. Even so, retailers do make an effort to buy only used movies that are playable and in good packaging.
A previously viewed movie was under the constant care of the video store and rented to numerous customers. This item was originally acquired by the retailer from a distributor in perfect condition and placed on shelves for rental. One benefit of a previously viewed movie is that it has been inspected by a store employee after each rental. If you buy it, you can be reasonably confident that the movie will play once you take it home.
Consider the difference between a used and previously viewed movie before your purchase, especially if you plan to buy online. Although the reasons a retailer labels a movie as used or previously viewed are clear, not all retailers adhere to the same standards on the condition of the items in stock. One retailer may carry DVDs that play perfectly but are scratched on the playing surface and stored in slightly damaged cases.
Most reputable retail outlets reduce the risk of your purchase by offering a return policy. Familiarize yourself with the retailer's return policy before buying a used or previously viewed Blu-Ray or DVD.