Yeah, if this is your video or you've used this yourself, please report it with a save in. It's hard for other player to make reports just based on a video, though I understand in short how this bug occurs.
https://forums.taleworlds.com/index.php?forums/singleplayer.658/
This trick has been going around for a bit now, what it is, sell/buy changes the price in towns for related items, however resetting before you finish is acting as if you finished the transaction, but you didn't so you can keep buy/sell to crank up the price.
@Flesson19 you're a trade guy, do you know if this has been properly reported?