Vendors will instantly refresh their inventories every time the trade menu is opened. This can be done without even leaving conversation, just close and re-open the trade menu. However, this means that if you like something but do not buy it, it will disappear the next time you try to buy it.

A weird interaction I noticed is that upon leveling up, vendor gear may not also level up (I was forcing level ups though so not sure if this happens on valid level ups). One workaround is if you save and load again after leveling up, vendor gear DOES level up with you, so that's not too big of a deal.

Items you sell remain in seller inventories even after multiple refreshes.