Calculus was created at the same time by Isaac Newton and Gottfried Wilhelm Leibniz. Some people still have arguments as for who actually was the first one, but we’re not going to tackle that issue because it’s just beside the point. In a nutshell, calculus is a mathematical study of change that deals with derivatives and integrals. The value of calculus from a practical standpoint is that it allow for predicting the future. We provide buyback of Calculus for best prices. Selling calculus textbooks to us is easy - just ship them for free and get money fast.
Speaking of practical application of calculus, you can use it in multiple areas, ranging from rocket science, anatomy, medicine to engineering, business predictions and weather forecasts. Thing is, calculus is applicable wherever you need to predict the future by calculating instead of running actual experiments, which can be overly expensive or just impossible.
For example, Being used to run actual costly experiments to figure out what shapes of fuselage would work better for their airplanes. Now they use calculus to figure that out without actual physical tests. That allows to cost-efficiently identify if a theoretical idea makes practical sense and can be use in real life conditions.
You can sell your calculus textbooks to the folks out there who are doing their courses to become doctors, engineers, astrophysicists and the list goes on and on. As a matter of fact, everybody who needs to predict the future backed up by numbers will need your textbooks on calculus.