I think it would be more realistic, if cheaper items, like small health potions and other "mainstream" items will respawn quickly, but the more unique/expensive an item gets, the longer it'll last till it/or an equal item reappears!...I think this is more real, because the merchant will probably have some items, wich are often sold, back in storage, and it'll take hime only a few minutes to get some new, but he has to order more special items (for ex. he has to ask the smith for a more special axe...and wait until it is made)...so it'll take some time until these items are reaviable!