Items available at merchants will get replaced from time to time. Find a vendor and equip all your equipment for raising your barter skill, then press F9 to quick save the game. If you do not see any desirable items, press F11 to quick load the game. The shop will open again with completely different items. This is recommended when shopping for specific potions or scrolls. Save the game if your choices are not that bad in case you do not have better luck with further reloading.