Online sales of used books, music and videos increase every year. But more than ever, those sales happen at fixed prices on Amazon Marketplace, instead of eBay auctions. Recently Amazon has brought more buyers to its used marketplace, most of whom prefer transacting immediately at fixed selling prices and standard shipping fees.

Amazon has grabbed an increasing share of used book, music and video sales because used items from Amazon's third-party merchants are displayed right alongside Amazon's new products. So casual shoppers browsing Amazon with the intention of purchasing a new book for $18, for example, notice that a used copy is offered for $8.

To be sure, a significant portion of used book, CD and DVD sellers still prefer eBay auctions. If a seller has a full collection of one author's books, for example, the collection can often be sold at a higher price as a package deal on eBay. A seller with one copy of each book written by Stephen King could sell the whole lot for a much higher price than the books would sell for individually at fixed prices on Amazon.

And many booksellers continue to believe that single copies of truly unusual, highly collectible books still bring a far higher price at auction on eBay, compared to selling at a fixed price on Amazon.

Nonetheless, most online sellers today maintain active seller accounts on both eBay and Amazon, to avoid putting all their eggs in one basket.

How much are your used books worth? Today it's relatively simple to determine their value, since most used titles are listed for sale online. For commonly available books, an average of the prices displayed on popular venues such as Amazon.com and eBay will provide a good indication of the market price. Supply and demand rules. If you price your copy much higher than competing offers, odds are you'll wait a long time for a sale.