This is a very unique idea. You can earn money by creating and selling your own T-Shirts! All you need is a photo editing software for creating designs, and a little investment on your part. Create interesting event-related t-shirts, and prompt people attending those events to buy them! For example, you can create shirts for seminars, social events, parties, etc. It’ll take some money to initially produce those T-Shirts, but you can sell them at a profit to gain more than what you spent! A friend of mine started with this idea, and now, she has a small business and a brand with a website, where people can choose from the available designs, or order a custom-made T-Shirts according to their desires! Great huh? Who knows it could evolve into a full-fledged business for you?
If you have anything you don't use anymore, and it's too large to sell online, Craigslist should be your go-to place to sell stuff. In college, you may want to sell all your furniture before moving home from school at the end of the year. Depending on what you own, a few $100 might be better than storing it or transporting it back with you. It's easy to setup a listing and sell anything on Craigslist.
Search for freelance writing and editing gigs. The rates for these jobs vary: you may be paid by the word, be offered a flat rate for a project, or in some cases can be paid an hourly rate. You typically won't be able to retain copyright over your work or collect royalties, however. Even so, by doing freelance work you can build a portfolio and make valuable connections which may later pay off with more stable job opportunities.