You can resell physical materials -- paperbacks, CDs -- when you are finished using them. This is a great way to stretch your homeschool budget. Many people resell their materials on eBay, Craigslist -- even on our Buy/Sell market on our Community forum pages.
Digital materials are a different matter. Because PDFs and MP3s are easily reproduced and duplicated, there is no way to prove that the original owner didn’t keep a copy. For this reason, selling digital products that you have purchased is discouraged, and is generally illegal.