Value includes clear and detailed product knowledge.
On many occasions we have been able to successfully price books higher because we took the time to discuss them competently. If a customer wants to deal with another person who understands their desire to purchase this particular book, an excellent listing can not only drive the sale but increase value.

Good used books will not retain their value unless they are truly rare. The values of good used books have a very short Internet shelf life.
Both Amazon and Alibris allow you to look at your older listings and compare them to current prices. It is shocking that books competitively priced a few years ago have entirely lost all their value because of the increasing number of listings and sellers entering the marketplace. Therefore, we routinely re-process all of our older listings and we've pretty much stopped buying general stock, focusing instead on rare books. If we had a bricks-and-mortar store, however, our strategy would be different.

In addition to current market pricing, values derived from previous sales can serve as benchmarks.
Internet book sales are not like auctions; they are very much "in the moment". Historical book sale data, however, is very important in approaching auctions, as well as in avoiding major listing errors. Historical book data can also add weight to your pricing decisions.

While there are buyers for books all around the world, in general, values are culture-driven. Books are worth more in the culture to which they relate; conversely they can be worth less in cultures to which they do not relate.
If listings are put on the Internet directly from local stores, without comparing pricing to the worldwide listings, there may be bargains to be found by buying from out-of-the-way locations. Conversely, prices are higher by selling in the market (and with the requisite language interface) of the book's author or subject area.

Values increase with the addition of value-added services.
Like many rare book dealers, we routinely search for titles. We also develop bibliographies and have a publishing company that creates custom books. These kinds of services are not affected very much by the rare book marketplace.

Values increase in positive feedback-driven environments.
Pricing can be less competitive if you maintain a high feedback rating on places like Amazon and eBay.