As the owner of a second-hand bookstore/book exchange, I can only recommend (nay beg) that you patronise these worthy establishments.
Why pay the extra money over and above goodwill? Well, firstly, any owner worth his or her salt will have the store set out in a way that you can find things easily.
Secondly, I can assure you that apart from antiquarians, few second-hand booksellers make much money - they do what they do for the pure joy of being surrounded by books. This means they'll generally know what they've got, and where it is, so you don't have to spend hours failing to find what you're looking for.
Thirdly, if they haven't got what you want, right now, they'll move heaven and earth to find it. These people haunt auctions and garage sales, publishers remaindered book skips, church, school and Red Cross fairs. They network, and can call on widely spread resources. They will buy a large lot, for a single book that they know a customer wants, because they can probably sell the rest.
Fourthly, because they are independent, you are likely to be dealing with someone who knows and loves books, not a mass-market shop assistant. If you like a particular author or genre, they should be able to recommend something else that will please you, and help you discover new treasure. If you've got time to spare, you can probably have a long, stimulating and fruitful conversation (and very likely a free coffee, and a chair out of the rain).
Finally, they'll often buy or trade in as well as selling - so you can still get new things to read, even when you don't have any real cash.
Support your local second-hand bookstore - or if it doesn't provide the things above, find one on the 'net that does -- I know a really good one *grin*.