Value, I've been selling on-line since 1995 and started with Amazon before Z-Shops, before Marketplace. What I have seen happening there is a travesty to book prices. When I am listing there, I routinely ignore all 1p books. What is more important, to me, is the number of copies being sold used, and the ranking of the book (found on the data page). I use Bookfinder, Addall, and ABE for pricing. I have gone so far as to order books advertised on amazon for 1p with the intent t resell them. All but one order was cancelled as unavailable. The one that was delivered was grossly misrepresented and returned. Which brings up another point.
As a newcomer, it is extremely important for you to describe your books accurately, completely, and in a professional manner. It is not enough to say edge wear to cover. Tell them if there is chipping, rubbing, scuffing. It is not enough to say Usual used book condition (I kid you not--seen just today on Amazon). You will get more sales at higher levels if you are accurate and complete.
You also don't want to load your stock with low end merchandise. Someone looking over your entire inventory will see an abundance of $2 books and will get the impression that is your schtick. I know it is tempting to list everything in sight, but in the long run it is not a good idea.
Lastly, since you are starting out.....put your books on a data base such as Home Base (free), Booktrakker or Bookhound. I use Booktrakker. You buy the program and don't pay a monthly fee. A lot of Amazon sellers use the monthly subscription programs. In the long run, the fees eat up your profits. You have to figure that 25% of your sale goes to the listing site with all the fees (commissions, listing subscription, postage reduction). You don't need another partner in your site. The book data bases are a one-time fee with life time support. (Let's keep Andy healthy, gang -- he's Booktrakker.)
Marilyn
http://northamericanrarities.net