Free For All
Many DMs let their players handle all their provisioning themselves, assuming they are in town, while the DM prepares for the game. The Player's Handbook lists items and services along with prices and players just buy whatever they need. In version 4.0, magical items are listed in the Player's Handbook along with the cost to create each item. (We'll save that whole issue for a future rant). Some DM's then let player's create any magic item they want on their own while the DM is busy getting ready for the adventure.
Realism vs. Speed
That is one way to save a lot of time handling a fairly mundane task. Because time is often quite limited, there is a strong argument for handling things this way.
But when is the last time you sat at home, decided you wanted an Egg McMuffin and it suddenly appeared? You at least have to go get it. If you go to Fred Meyers and ask for an Egg McMuffin, they are at the very least going to look at you funny. Not only do you need to go get what you want but you have to know where to get it.
The D&D world is no different. If you want some torches and rations, you will need to know where to go. There may be multiple locations that sell them and each may have very different prices. Unusual items, and even common ones, could be out of stock or not normally carried. Some items may have very different price tags dependent upon what town you are in. Certain items may only be available from a sole source that players must travel to if they want it.
Walking through a D&D town can be an adventure by itself as well. There are pickpockets, assassins, damsels in distress, dark figures following you for various reasons, strangers stopping you and begging for your help. Many of these make great adventure hooks to get the evening started. The point is that all of this background is lost when players simply order what they need from a catalog before they leave town. It does cost some time but there is some benefit to running in-town errands as mini-adventures.
What Is Available
At the very least, tell your players that they cannot buy from the lists in the Player's Handbook. Create your own list, for each town if not for each store in each town, of what is available and review the prices yourself.
In my opinion, the prices listed in every version of D&D so far have been ridiculous. Consider the level of technology, the availability of magic, the cost of having spells cast, the difficulty of transporting goods between towns, etc. Then you have first level characters coming back from a week long adventure with enough gold to rent the best rooms, buy the best clothes, hire entertainers and throw lavish feasts every day for a year. The entire economy is out of whack. I will write a lengthy article on economy soon along with my own revised price lists that you can choose to use, toss, or modify as you see fit.
Beyond the cost and availability of supplies, you also need to consider NPC services. PCs will need the services of Clerics specifically. From time to time they will need to have a curse removed or a friend brought back to life. A small town may not have anyone around who can do that. Some clerics may not be so anxious to help a character who does not follow the same deity which may affect the cost of services or whether the Cleric is willing to help at all.
NPC Wizards are always bound to be busy doing other things and won't want to be bothered at all by PCs who want to have spells cast or items created. It may well require that the characters perform some quest for the wizard before services are even discussed. Costs should never be cheap.
Even mundane services such as blacksmithing shouldn't be taken for granted. Just because a warrior decides he wants a set of Full Plate armor doesn't mean that there is a set just hanging on the rack waiting for him to buy it. You may have to order it, pay in advance and wait a week or so (or longer) to have a set crafted.
Availability of goods, or more specifically the lack thereof, offers some opportunities for adventurers. If one town has none of something but another town some distance away is known to have some, perhaps the adventurers can make some quick cash playing traveling merchant for a bit. Additionally, this can be used as a method of driving characters to another town where you have an adventure hook waiting for them that will lead them to a new dungeon you want them to explore.