I've been through this with education tech spending. We couldn't source our own small or consumable items, we can only use the authorised suppliers - not shop around, nor pop into a local shop and talk them into a good price. And absolutely not the supplier we used to use most often, who are owned by the same company as the authorised company but actually charged less.
And when we asked the IT buyer for purchasing advice on bigger items it was pretty obvious he hadn't a clue about TCO or even the qualities of other brands than the ones he always ordered. It was always Brand Y and one of three models (big medium and small, in effect) whether this met our use needs or not.
AND ( screams) we had to pay the buyer's department a percentage on top of the cost, so to our budget it was even more expensive and to the public a total waste of cash that was meant to be used for us to serve them.