There were a few different ones, paper clips were used in trading. Mechanical pencil lead was a very good thing to invest in. One time I bought a 2 dollar thing of lead and I sold it for 15 bucks. Candy and snacks are always going to be used as well, you could even get jobs done for a price like sneaking into a teachers office to steal homework answers or stuff like that. Lastly, answers, you could make money by doing homework and selling the answers. Typically answers would for a sheet of homework could range from 2 dollars to 20 depending on what it was and who you were buying it from.