New books can be expensive, and if you think the prices in the USA are high you should see what they are charging in Europe. But once having read them people often either give them away or sell them for a few cents.
Buying second hand books is easy visit any flea market, thrift shop or charity sale and you will find plenty to choose from. The most saleable books are non-fiction hardbacks which should be in a good to reasonable condition. You may be able to get a good price for a slightly dog-eared copy if the subject of the book is good but ideally you should be looking for as near mint a copy as possible.
Many book second hand booksellers specialise in a specific subject area or niche and this is a good idea. If for instance you specialise in military subjects you will be able to build up a list of customers and offer them further military books as you acquire them. The secret of success in the second-hand book business is to have a list of customers who you can make regulars offers too. In fact many of your customers will send you their wants list of books.
There are plenty of places on the Internet that you can offer your books for sale, www.alibris.com...