How to keep momentum, especially during downtime.

I'm interested in this topic, but I also have kinda the opposite problem: I want to run compelling downtime, but I always seem to wind up DM-ing for players who just want to get through their shopping lists and roll their checks on whatever crafting or researching they're up to and then move onto the adventures at hand.

I want to figure out how to make downtime less "Rolling my persuasion check with the shopkeeper and getting my stuff for 15% off" and more "Role-playing with interesting NPCs". I want stints in town to turn into their own interesting mini-adventures where the party Rogue steals from the wrong person and winds up embroiled in thieves guild politics, or the Wizard stumbles upon a priceless volume of arcana in the shop of a bookbinder who doesn't know its value, or the Fighter is a little short on funds for the magic sword he wants but can get a discount if he scares off a rival for the smith's sweetheart's affections. Like, maybe I'm the only one, but running dungeons and encounters with unintelligent monsters isn't where I have my fun when I'm DM-ing.

Way I've gotten around is I have the players be responsible for shopping while I just control the NPCs. In some editions, there charts upon charts of stuff, and a first the idea of rolling up a shop sounds nice until after 45 minutes you've got a two magic battle axes, 4 handkerchiefs and a master-crafted iron spike (HOW ARE YOU A LEGITIMATE BUSINESS IN A TOWN OF 12 PEOPLE??). I say to the players "so you have all these books too (yay srds), flip through them before the game and see if there's anything want, write down the prices and go from there. Maybe if you buy enough they'll be a discount." Meanwhile I'll have like 1 or 2 specialty items per shop (cheap health potions but they don't heal until 1d4 turns later, etc) that may encourage rp when they go to drop off their lists. The other thing is they have to figure out where the shops are, maybe if they roll low they'll get a place that will over charge them ran by a shady NPC, but it's the only place they could find on short notice and maybe the next time they need to get in contact with the underbelly of the city they'll have a place to start.

Similar to Traveling, I question if downtime should keep the momentum going. Especially in a town when the adventurers return, it can be a rewarding change of pace. If your want to keep it interesting, which at least for me, is something different than keeping the momentum going, you can always deploy some unique NPC or Miniplots, that are going on in some shops.