This is a known issue, one of the most viewed topics was regarding this:
Unfortunately Blizzard and forum mods decide to ignore and bury the issue, this sudden GPU temp and power usage on vendors and in turn towns has been proven on many threads, yet they turn a blind eye. Ignore Unknown posters they are trolling.
The main thread where Blue chat Blizzard staff themselves said they were combining posts regarding it, rather than collating data and addressing the issue, gathered all threads into one, ignored it for a month, then closed the thread.
They have no either desire or ability to fix the problem, despite many people offering a workable solution of a selection to turn animated vendor models off, perhaps they are just too inept.
I would suggest lowering your graphics to high or less, turn fur to low, cap FPS at 90, and turn Nvidia reflex off. Only then you may get more manageable fluctuations, but will not fix the problem, so attempt to go in your inventory and vendors as little as possible - despite that being one of the main parts of an item grinding game.