Yes you need to join a community where you can buy (used) and sell homeschool books.

And never write in a workbook (do the problems on a white board). If you want to keep something like a unit test or end of year test for a portfolio out of a consumable book photocopy that one test for use so you can keep it. Then you can sell the work book. It isn't fair that you spend much money on something that you have to force the kid not to do the whole thing in one or two sittings.