A dime a dozen
One of the most prevalent items I find at yard sales is books. Cookbooks, Fiction, Mysteries, and Children's books seem to be set out in boxes to pick through all the time! I can count on one hand the number of times I've been to a yard sale where there weren't any books. Many people buy them new, toss them on a shelf after reading them and forget about them. You know until you need space and they are easiest things get to weed through. Then someone like me comes along and picks them up for practically nothing.
When I was teaching, this is how I would fill my classroom library with personal books without breaking the bank. Now I just fill the library in my own home. Books are always something I will invest in for my kids.
Thankfully, Bear loves to "read" and be read to. He is learning to sit and listen to longer stories when I read them, but right now he loves the easy readers when on his own. I love that he will seek out books on his own so I have no problem having shelves of books he can pick through. When we run out or he's looking for something we don't have, that's what the library is for.
Kids aren't little for long and I really like reading aloud to my kid even as they get older. So I have no problem picking up some books meant for older kids. If they don't like them then we resell them and no harm done. If they are in good condition I sometimes try to donate them to local schools. I am shocked more schools don't have programs that allow for book donations as I have had schools/Teachers tell me they don't do donations.
I try to keep a variety of books for the kids and allow them to move up if they treat them well. I believe kids need to treat books well. They aren't for chewing, coloring, ripping, standing on, etc or any of the other things the average kid will do to books. They need to be respected. Thankfully, this is easily done with some reminders from us. They were super cheap yard sale finds but they still need to be treated well so everyone can enjoy them.