Rarity checks

I'm putting together a SotB campaign after being away from Genesys for several years, and have been thinking a bit about shopping rolls against rarity.

The game doesn't suggest a frequency for how often someone can try to find an item for purchase. How have other adjudicated this? My gut says not to even roll for items that might call for a simple or easy roll, unless there is some other constraint (e.g. time pressure). For anything more difficult, look hard at the narrative situation?

I don't want to devalue Negotiation or Streetwise as skills. I also don't want to have a complicated minigame every time someone wants to buy some interesting gear. And I want to be consistent for the players.

So I'm curious how others implement the rarity mechanic when it comes to shopping. Can the PC's roll once per session per item? Once per downtime? Once per day?