How to do shopping

The shopping window allows the player to both buy and sell items. This is an important dialog window. Making errors while buying or selling could cause some serious damage to the player. But on the other hand, players will use this dialog quite often for annoying but unavoidable routine tasks like getting rid of vendor trash or stocking up on stock consumables.

So how should I design this from a usability standpoint?

The one-click solution

I put a "buy" button and a "quantity" input field (default: 1) next to each item in the vendor menu. While the menu is open, a "sell" button with a "quantity"(default: current amount) is added to the item detail area.

The drag&drop solution

Items are bought by dragging them from the vendor window to the inventory window. Items are sold by dragging them the other way.

While this is pretty intuitive and places the bought items right where the player wants them, it makes it hard to buy whole stacks or sell partial stacks (maybe ask it with a popup window?). It's also hard to communicate how much money the player will make with selling an item.

The bargain solution

This would mostly use a dialog I plan to use for player/player trading. The player drags the items they want to sell or buy from their inventory and the vendors inventory into a box. The sum is calculated on both sides and the difference is listed below it. When the difference isn't larger than the money the player got, the buy button becomes active.

This especially makes it more comfortable to replace equipment with more expensive one.

The Tibia solution

No, seriously. That's ridiculous for both usability and immersion. Whoever thought that using a chat parser to do NPC trading in a MMORPG would be a good idea, was either lazy, stupid or both.