Unless it has changed quite recently, inventory restocks every 90 minutes of real time that you aren't in the same screen with the merchant. However, with the game speed acceleration ability, you can significantly cut that down, I think - you certainly could using CheatEngine to do the same thing.
As far as savescumming it, I dunno. Styg has been pretty aggressive in reducing randomness in the game, for the last year or so. I believe you can, but I also haven't tested it. You could! Just check a mechant inventory, then save, leave the screen, speed up the game, and go grocery shopping or cook dinner or take a walk or watch some TV or something. Come back, see what's there. Nothing you need? Take a screenshot, reload your save, and repeat the process. Take another screenshot, and compare the two. I believe they'll be different.
