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Sell your contacts
There are a lot of services out there which offer "affiliate marketing", i.e. you refer your friends (who you know have certain needs) to certain products or services, and when they buy them, you earn referral commissions.
For example, do you know programmers? Make money by referring them to Codelicensing.com:
Selling used books can be even more profitable then selling new ones. If you have a rare book or a book that is too old, selling those may be a good idea. Even if it is not a rare one, if you know how and where to sell it you can make money.
Researching the web is a perfect way if you want to find the best price. BookScouter and BookFinder are two websites to start with. Here you can figure out how much money you can get from this or that company.
You can also start working with book-dealers such as ValoreBooks and Chegg. You can sell via AbeBooks and Amazon. All you need to do is to write info about the book (author, title, year of publishing, etc.). Finally, you can sell them at your garage or suggest it to passengers at metro stations and on streets.