Amazon.com has made increasing use of trade-in programs this year, letting people swap their used video games, DVDs and Blu-ray discs for Amazon gift card credit. It's a clever strategy, given that many consumers are reining in their spending and looking for deals in the tough economy. Now Amazon is expanding trade-ins to another big market: college textbooks.

Textbooks are big business. The top four college publishers did more than $4 billion in annual sales last year, according to one estimate. Amazon is targeting the textbook market with its Kindle DX reader as well, positioning the device as a replacement for physical textbooks at universities across the country (with mixed results so far).

The textbook trade-ins work the same way as video game, DVD, and Blu-ray trade-ins at Amazon. People look up titles online, print a pre-paid shipping label and mail it, and receive Amazon gift card credit in their account.

Amazon didn't specify how many used textbook titles it's accepting for trade-ins.

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