Lots are groups of cards that are sold for one price. Lots are advantageous to both collectors and sellers. Why? They tend to be the best deal. Buying multiple cards at once will allow you as a collector to build your collection faster, and as a seller, lots will allow you to maximize profits.
Purchasing single cards on eBay can be expensive if they are not of extremely high value. While you may find auctions that can be won for as little as a penny, once shipping is added in, the cost could be several dollars. Compare that to lots, in which shipping will still be a few bucks, but for many more cards, and you will see the value.
There are many different types of lots. While some are made up strictly of base cards, there are plenty that contain inserts, parallels, game used, graded, autographs and more. Be on the lookout for high value lots if you wish to purchase them for re-sell purposes. A great example of higher quality lots listings can be found here: eBay Sports Card Lots
If you are tired of the nickel and dime + shipping game, look no further than lots. They are simply the best value for your collecting dollar.