There are quite a few places on the Internet where you can buy and sell used homeschool curricula.
A library is a valuable resource to which homeschooling parents can turn.
It helps to either use a curriculum that you can photocopy the pages of the worksheets, or you can print off multiples from a PDF.
Buy your curriculum when publishers and companies are holding sales or offering deals.
A digital version will also save you money if the physical version of the curriculum is consumable.
When you’re done with your old curriculum, sell it!