If it's a common item with a listed price, and you're in a city big enough to reasonably have that item in stock, just do your shopping "offline". Sometimes I even include a low-level Forge cleric in small towns so the party could do their sub-100gp item shopping. (In that case the cleric charges an extra 10% donation for the Forgetemple, which they will use to feed orphans, create farming equipment, etc...)
Yup this is how we do it, don't count anything lower than gp either. Game time passes but not real time, and if you bargain you make corresponding roles, DM can decide if you get advantage or disadvantage depending on the context.