This is the way that i buy and sell books,   hopefully this will help someone else make a few bucks from selling books.

Buy cheap and buy good condition books, i usually purchase books at garages sales, auctions, and resale shops, every now and then i am able to puchase books from local libraries.  I never pay more than a quarter for a book, simply becuase i may only sell it for 75 cents, hey that is still a 300% return! 

sometimes at auctions i get boxes of books for only a few bucks!

buy books that haave an isbn number on the back cover and try to buy newer books, i made the mistake of buying a bunch of textbooks that were no longer being used in colleges.

textbooks, you are able to get a premium for good used textbooks, always keep a look out for these books,

i only sell my books on half.com but some people sell on amazon and have had success.  (you might also try duperoo.com)

do not buy or try to sell books that are ugly, have water damage, or lots of markings in them, these sells can hurt your feedback rating if you do not describe them properly. 

I keep a bookshelf just for my half.com books, i keep them on the shelf until they sell and i just take them off the shelf and ship them. 

Every 3 months or so go through and remove a few old books, make your stock look new, box some of your books that have not sold in a while and take them to an auction (to get a few bucks back) or donate them to a charity (great tax break)

I go to college full time, work at a retailer part time, and run a multinational online company in what little spare time i have, but when i open an email and noticed that i have sold a book, it is like Christmas, i get excited..... 

The market is always changing for the books that i carry, one month i might sell one book, the next i might sell a book a day.

Books that sell well are biographies, religious, and textbooks (only in fall and spring)

Hopefully i was able to impart some new knowledge to you, if you have anything to add or any questions, please contact me!

Remember that customer satisfaction is the key to repeat buyers, i try to ship as fast as i can, and sometimes i try to include a free bookmark with the purchase of 2 or more books,  those little things go along way!

like i said, i hope something in here was helpful to you.

UPDATE ____________________________________________________________________________-

I am now selling on Amazon, they take a higer commission for each book sold, but...  you get more for the books than on half.com,  I am very impressed with the ease of listing on both sites.  I like amazon because I get more $$$$$ per book, and I like Half.com because I sell more books!