If I give my parts supplier a resale certificate, isn't everything I buy from that supplier exempt?

No. Suppliers must exercise reasonable judgment in accepting or acting on resale certificates. They must charge the tax on sales to repairers of goods that are obviously not for resale, including tools, stationery, soap, masking tape and sandpaper, even if repairers have provided a resale certificate. If suppliers fail to charge the tax where applicable, they can be held responsible for the uncollected tax.