You're missing the point
One thing most of you are not considering is the value of your time and the affect random buying can have on a business. If you need to go out to local shop to get something you are not doing your job for an hour, when you come back with the receipt someone has to process your expenses and split the VAT, someone has to deal with going back to the shop if the part fails in six months time, someone has to sort out the problems because the desktop roll-out image no longer works as you've bought a card with a different chip-set, someone has to ensure they can get spare parts in five years time. The list is endless.
You are not paid to be a shopping assistant. You ask for a part, [in theory] it magically appears. For a business it is very difficult to keep track of items bought ad hoc.
Looking just at the raw purchase price rather than the overall cost to the company is naive and blinkered. Yes, there are many instances where it can go wrong and you can point out where there are problems, but overall buying like this can have a lot of benefits to people looking at the overall effect on the company.