We do reuse backpacks. We got Pottery Barn ones and they were pricey but last 3-4 years. The lunchboxes last too, but we usually replace before the backpacks bc they will leave them somewhere with stuff in them and they get disgusting. We got my youngest one of those zipit pencil boxes last year and it's still in great shape. So if he's ok with it, he can reuse it.If the supplies are in good condition we will reuse them--binders, scissors, rulers, 3/4 blank notebooks, glue bottles over half full, supply box, pencils, erasers that have not been split in two with a pencil (why do they do that?!?)
My oldest seems to always bring home 5 brown markers, three green and a red--never the full 12 count rainbow he started the year with, so markers are always picked up at the start of the year.
We're also meanies who make the kids use the same backpack and lunchbox as long as there are no rips/tears/weird smells.
You got lucky. My 3rd grader has a list that includes a blue pocket folder with prongs and an yellow pocket folder with holes. Oddly specific things, that don't seem to hard to find, but I made the mistake of waiting till a couple weeks before school when my oldes was in 3rd and couldn't find the specific folders anywhere... There are about 20 things on each of their lists. Nothing extravagant, just what I'd think of as normal stuff. Pretty much the same as when I was in school 30 years ago. With the exception of headphones/ear buds. Didn't need those lolDD's school never sent home any "required" supply lists. Sometimes a teacher would request a binder or folder. Binders we often re-used maybe with a fresh pack of 3-hole paper. We have plenty of pencils around and made sure she always had a few in her bag or a pencil box. Other stuff like glue, markers, colored pencils, etc. we just had around home for when it was needed for homework. High school did require a specific calculator, but shared a link where it was on sale and also mentioned an older model would be acceptable.
Things like pencil boxes, if they are still in good shape I will ask if they mind reusing them. We do always get new crayons bc they get broken.
When did trapper keepers get to be $30? Holy cow. That is on my 6th graders list. He needed one in 4th grade too, and went thru 2 of them in a year.