Merchants forever keeping all items ever sold to them may sound good on paper, but unfortunately it becomes a massive issue real quick that ends up crippling the game's functionality and even performance if the player enjoys exploring, looting and then selling that loot to their favorite merchant in the game.
The issue is that the game encourages building a positive attitude with a merchant by gifting them stuff, which then reduces their own prices and increases the value of our own items, so naturally selling to our favorite merchant with maximum attitude is the logical thing to do as doing so maximizes profit and minimizes costs.
Unfortunately this creates three major issues;
Browsing through a merchant's inventory becomes a huge inconvenience due to all of the sold items that the player ever sold to the merchant
Pickpocketing drastically suffers and becomes inconvenient because the player has to go through 2000 of their own items to find the one they want to steal
The vast amount of items that are never cleared out start causing massive frame-drop stutters whenever the player gets within the rendering distance of the merchant, because it loads all their 2000 items whenever they're rendered.
Here is an example of my two favorite merchants Dammon and Arron. Their inventories are so massive that every time my character comes within their rendering distance the game just takes a massive dump on the framerate due to 2000 items being loaded each time.
Because of this the Druid's Grove in my playthrough now exhibits 5 different massive stutter points which drastically drop the framerate each time Arron is rendered into view. An even worse case is with Dammon in Last Light Inn because he is constantly moving around, so he keeps causing massive stutters every few seconds while the player is inside of the Inn itself since that's exactly the border of his rendering distance.
It would be really nice if this could be addressed and optimized by having sold items clear out after X amount of rests, for both convenience and performance sakes. There is simply no need for merchants to indefinitely hold items sold to them, as the whole point of selling items to them is to get rid of useless junk.
