I'm the Equipment Coordinator for my troop and I can tell you that we handle things in a variety of ways depending on the item.
For most equipment, I wait for the SM to tell me there is a need, then I go find whatever the need is at the lowest possible price. If I happen to see something we could really use, but maybe don't absolutely need, I'll send a quick email to the SM, CC and Treasurer to see if anyone wants to veto and if not, I'll buy it. If I happen across a fantastic "till the end of the day" price, I'd probably just buy it on my own "authority" and hope no one pitches a fit.