Value is relative to the market in which you are selling.
Books with absolutely no value on ABE, for example, may not face the same competition on Biblio or ZVAB. You just have to decide whether it is worth the extra effort to check other markets to finalize your price.

Competition diminishes prices; therefore don't bother selling titles with huge print runs on the Internet.
Give books like this to your favorite charity: Lee Iacocca, Talking Straight. New York: Bantam, 1988. First edition, fine condition. $2.00. 169 copies in various states are currently on ABE. Anyway, would you rather sell one book for $200 or 100 books for $2.00 each?

High-priced books make other high-priced books look more valuable.
Would you rather buy your $100 book from a dealer with many other similarly-priced books, or from one with bargain-basement prices? Customers have to have confidence in the value claimed by the dealer.

Values increase with interesting catalogues of related titles.
Even if you're selling on eBay, it is better to sell similarly-valued books with related topics together, than to mix them up with less expensive and unrelated titles. There is a reason that rare book dealers develop catalogues. The longer customers spend with your catalogue, the less likely they are to seek your competition. Intelligently-written catalogues increase perceived value and lead to less competitive sales of related titles.

Value includes safety when purchasing on the Internet. Customers will pay more to ensure a safe and worry-free transaction.
Amazon is an excellent example of this principle. They sell a ton of new books at higher prices, even though Amazon sellers undercut them like crazy. Customers know that by buying direct from Amazon they're not going to have any problems. Problems take time and aggravation to resolve. Problems cost money.

Value includes speed and ease of purchase.
Customers are not driven to go deep into a web site to get a better price. I performed a personal experiment on this with a book I wrote: Paul and Renee Magriel, Backgammon (2004 edition). Harwich Port: Clock & Rose Press, 2003. I set up listings on Amazon's highest level page, as well as a significantly lower-priced listing on Amazon's z-shop, where I paid a much lower commission. I would say that 99% of my customers did not seek the lower-priced listing because they had no obvious incentive to go beyond the simple search that turned up the top-level page. With new books, this is also true if one compares Amazon to other commercial or private sites.