Buying cheap books is not really terribly difficult these days particularly with the advent of the Internet which makes it very easy to use search engines to find just about any type of book, author, title; even the rarest and most difficult to find books are only a few moments and a few clicks away whether you are searching for new, used or even very rare books.
One of the biggest advantages of online book shops is of course the fact that you can do all the selections from your own armchair and the book will be delivered to your home, which means that you do not have to spent hours searching through a bookstore, even if you have been lucky enough to find a parking space within walking distance of it! The way in which books are normally sold is that initially a cover price is decided upon, which is the price the publishers think they can get away with. Profit margins to retailers tend to be quite high which is why online booksellers like Amazon can afford to give such decent discounts off these cover prices. One of two things happen; either the book sells well or it doesn't, and if sales levels are poor the books may be remaindered and sold off at a low price. There are a number of specialist booksellers who buy up these remaindered stocks and so they are able to offer them at very cheap prices. If the books do sell well they are reprinted in due course, but there are always a number of the older editions still left on bookshelves. Most publishers work on a system called sale or return; this means that booksellers who cannot shift all their stock send them back to the publishers for full credit. Some of these books of course may be soiled or damaged and therefore cannot be sold at full price. Again, these are bought up by specialist retailers who can then offer them for sale at really cheap prices.
Books follow the same law of the market; supply and demand. If they sell well they will keep their price and if they do not sell they will probably be available for a much lower price. Just because a book does not sell well however does not make it a bad book; it may be that the subject matter is only interesting to a small proportion of the buying public. This can be a double edged sword because sometimes the publisher will print too many, and be forced to discount them; sometimes they will only print a few and demand will exceed supply. It is not unknown for some second-hand books to fetch many times the cover price that was charged for them when they were new.
The bookseller Amazon not only sells books itself but also passes orders on to other more specialised stockists and you can often get some very excellent bargains from these companies, particularly if you are willing to read a book that someone else has already read. Many of these companies that sell second-hand books will give you a fair description of the amount of wear and tear that the book has suffered.