I am interested in bulk buying of used/new books for further sale on
internet through places like Amazon Marketplace, alibris.com,
abebooks.com etc. Where do the sellers of used books get there books
from? Which subjects of books are fast sellers. How much does these
sellers may be earning per month?

This might be helpful:

http://www.stretcher.com/stories/01/011203k.cfm

Libraries often have book sales. Our city libraries have several
shelves of used books, that are constantly refreshed, as they weed the
stacks. Prices are excellent, 25 cents to a few dollars.

Used books can be purchased at yard sales, and flea markets. A good
stratagy is to pick out a dozen or two books you think you can resell
and say "Will you take $10 for these?" Typically the seller will make
a counter offer. You need to double your money most of the time, or
you will be working for $1 per hour as some books won't sell even for
25 cents. If you want to buy books in lots of 10,000, you need to find
a local seller who wants to reduce inventory, or needs quick cash.
Shipping will typically eat up your profits on used books. You are
competing with ebay and amazon.com, plus big local retailers, so you
should probably not sell books as your primary sourse of income. It
will take a long time to learn what can likely be sold at a profit. 
Neil