While my players will easily kill 30 minutes collaborating on what to buy, a session like this is definitely more fun if there are NPC’s involved.

We had a memorable session when the PC’s found the Emporium of Evil, where they tried to find the magic items that weren’t TOO cursed, speaking to all manner of morally questionable merchant. (They bought a lot, actually.)

You can also brainstorm the next quest this way. Whether or not the party wants to take a quest from a one of these merchants, they can certainly hear rumors. You can see what they take interest in, and build your next plot arc off of that base.

Some trouble can always pop up when the shopping is winding down, requiring decisive action by the party.