It's pretty common for items in merchant shops to be
extremely overpriced compared to how much they usually cost in NPC shops. This is a problem because NPC shop items are required for quests, which forces players who aren't stocked up beforehand to either A) let their quest time out, which can take hours if it's on a low tier, or B) pay the severe upcharge and pray that the subsequent quests will make them that TU back. Even when an item's merchant shop value seems stable, there's a constant risk of a user buying out all of the reasonably priced listings so they can try to sell it for a ridiculous amount.
This isn't a new problem mind you, but I feel it's a more pressing one now that most players aren't going to be making hundreds of millions of TU in one day off of quests alone. The chance of making back the TU spent on an overcharged item is a lot slimmer than it was just a month ago, especially when multiple items for a quest are being overpriced.
To preserve the profitability of quests, I believe that NPC shop items should have a price cap when stocked in a merchant shop. Personally, I'd suggest anywhere from five to ten times the average NPC shop price, since that seems to be the range of TU quests give back per item.
I feel that the Costume Shop, Black Market, Ty's Dyes, Cain's Hoard, and Twuntie's Treasures should be excluded from this cap though. None of these shops have items that are used in any quests, so there's no alternative way to make a profit from any of them. In addition, the latter three use RC, which could lead to issues with the cap if the value of RC ever changes, and the Costume Shop's seasonal items might be difficult to cap since they don't have a chance of being stocked year-round like other NPC shop items.
In the instance that an item is permanently removed from an NPC shop for some reason, the price cap should be lifted for that item, since merchants and trades would become the sole suppliers of the item and the item presumably wouldn't be asked for in quests anymore.