Be it rumor dispensers or loot drop-off points (or even just a "trade" button attached to some NPC or other", I think merchants and shops could stand to get some attention and personality.
While I guess there won't be any "not buying x" types of deals you could have different things be worth different prices to people?
There could be some rules, traits or themes certain shops have, not even necessarily affecting the player, like a sign in front telling you to wipe your boots on the doormat before entering. Interesting shops like Sorcerous sundries, Lucky Aello's Discount Store and Adventurer's Mart come to mind.
There could be some advertisement going on for the bigger shops, like the criers in the BG line. Or the merchants could just be shouting at you when you pass their booth.
Going from there, merchants could have some comments on the stuff they're selling, beyond just "my goods are awesome". Perhaps when you click on the fancy flaming hammer you could get a line or two about where the merchant got it or how he it can shatter a dragon's claw. There was a shop with something like this in PS:T I believe? Same thing could be applied after you buy it instead. Or when you talk to him the first time he points you to the items that he wants to sell the most. Maybe in a different shop they try to push on you that overpriced golden adventurer gear.
How about some "scams", too. I'm sure you've seen some telecommercials with plenty of half truths and distractions to "technically not trick you" into buying stuff. Conversely it's possible that the dented worn mace you bought for cheap turns out to be a fairly good wand or something.
Some merchants could be into good customer service and offering discounts and deals for frequent partons.
I think stuff like this would improve the feeling of connection of the world and the economy, even without such extensive consequences as the iron shortage in BG.