When you attend college, you quickly realize how vicious the textbook game is. You first buy a new textbook for upwards of $150 (not a stretch) and then when you try to sell it back at the very same bookstore you bought it from, they offer you $25 if it's in very good condition. And if you pay close attention next semester, you'll see it for sale as a used book for $120. It's a money game you are not intended to win.
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The best way to circumvent this cycle is to sell your textbooks online. The Internet frees up the barriers to entry. First, you can easily have a storefront or means to sell. Second, you can ask for any price you feel your text is worth. If the market will bear that price, your book will sell.
Two of the best places to sell textbooks online are eBay and Amazon. eBay charges you a percentage fee based on how much your total selling price is. You will also need to pay a listing fee. Amazon charges you a fee set on how much you list your book for but there is no initial listing fee – only if it sells. The advantage of Amazon is it shows up better in search results and most people buy books from individuals with subconscious thoughts that Amazon is the seller even though it's obvious individuals are selling under the name.
You can also sell on public forums without the associated fees, but you typically won't get nearly the audience the aforementioned sites get. You will also have less trust level – especially as a newer forum member – and this has a tendency to lower the price. Amazon and eBay have established member rating systems for sellers. Some forums do as well, but many don't.