Roleplaying the shopping time
In this variant, players wander around the town, look for shops that sell what they want to buy and they ask you if they can find what they need. Character will most likely split up and be asking a lot about what's available.
Great occasion to plug roleplay and events. A kid just cut one of the PC's purse while 10 others run around him. Do you have taxes to pay? Why? Explanation could provide information about population satisfaction and may lead to a quest. Also you can manage the price, negotiating the price and also what is available and why. Great control over what the players can buy.
Party split up can be a problem. This variant takes more time.
Players buy, GM approves
In this variant, the players check into the book, buy what they need at the said price. They tell you want they got and you can approve or reject their shopping list.
Really quick so you can go back to the action real quick.
Players are limited to the list inside the books. It also assumes a single price, the one inside the book.
Equipment is bought outside game time
In this variant, game time is dedicated to roleplay and combat. Buying stuff is done between game sessions leaving the game sessions for the "real deal". You review the buying either by email or in person but outside the game sessions. In this variant, ammo and components are assumed to be available at all time or you can keep track of the spent ammo and simply pay the cost outgame.
Gives players all the time they want to shop. The actual game time is spent for activities related to the story or the progression of the game.
Cuts all interaction between shopping and the game itself.