I've been scouring all the undiscovered locations and have gotten into the habit of looting everything from the enemies I have killed and then selling the weapons and armor to Lafargue in Toussaint. I've accumulated over 200k coin so far doing this.
One thing I noticed is it appears that Lafargue has kept everything I have ever sold to him. Meanwhile there's an art merchant nearby whom I sell other junk to and I noticed that his inventory has cleared unlike Lafargue's.
I'm also pretty sure Hattori has kept the weapons I've sold to him since the beginning too.
So it seems like some merchants will hold on to their stock but other (lesser?) merchants do not. Is this the case?
The only reason why I ask this is because the menus lag a lot when there are a lot of items onscreen (on PS4 at least, not sure about other platforms), and it'll be a pain to deal with Lafargue in the future if his inventory will forever be chock-a-block of worthless weapons like longswords and blackjacks.