What is something people have to buy every so often? Something like furnace filters?

Batteries

AC filters, batteries for smoke alarm, flashlight, radio (a big one for me, I live where there are hurricanes), household cleaners, light bulbs, coffee filters, detergent, soap, shampoo and other toiletries, dish detergent, laundry detergent, bleach. I have to replace the belt in my vacuum cleaner every few years because it breaks or gets too worn and then the sweeper doesn't work right. The flapper in my toilet needs replacing every few years when the toilet starts to run.

Shoes.

New tires.

I'm assuming you mean home improvement type items that need replacement. Maybe light bulbs, water filters, liners like cabinet shelf liners, shower heads or shower head filters, fire extinguishers, smoke detectors (or the batteries at least), etc. There are also household items like sponges, underwear, furniture, and fixtures that need to be replaced

Condoms.