I recall many years ago when working for a county council that our departments were free to purchase from the best price you could find.
Until one year when they signed an agreement with a printer supplier and we had to buy all consumables via them.
These were vastly marked up and IIRC the prices we set at a fixed unegotiable rate at the start of the contract and renegotiated every blue moon.
It was so bad we ended up buying brand new printers that came with rainbow packs or black toner carts as it was cheaper than simply buying the new toner via their contracted partner.
In fact, I don't think I have ever come across a central purchasing department that ever managed to get value for money.