If you are a buyer or purchasing manager for business, which of the following ways do you buy MRO items (maintenance, repair, and operating supplies)?

Buy only when immediately needed.
Buy expected amount needed for a month or two.
Buy amount depending on cost and price breaks.
Allow suppliers to estimate our needs and buy and store that amount.
Have supplier store amount for us that we use in a reasonable time.
Store amount by item category depending on cost and usage.