Summarize:

If you can't make it to a concert or sporting event, the money you spent on tickets does not have to go to waste. You can also try buying a few extra tickets to an event at student prices. Then, sell them to friends and relatives who do not go to your college for an extra cost. If you go to a school with a big sports culture, tickets to things like football and basketball games are in particularly high demand. Many students resell old textbooks at the campus bookstore at the end of the year. However, resale prices there tend to be low. You can try reselling your books on your own. You may get more money going this route. The website BookScouter will give you an idea of how much a specific used book tends to sell for. You can then try to sell the book online for that price, or try to resell it to another student for a similar price. You can also go to local used bookstores. If you have a book that's in quality condition, you may be able to resell it there. If you just got a new phone or laptop, do not throw out the old one. You can resell old electronics online or through a used electronics store. People may need the parts, or be okay using a used laptop that's less pricey than a new one. You can auction off old clothes on websites like eBay. You can also stop by a local thrift store or secondhand shop and see if you can get any money for old items of clothing. You can also look for online consignment shops, like ThredUp.
Sell tickets to events you're not attending. Resell your textbooks. Sell your old electronics. Sell your old clothes.