It has come to my attention that some sims have older vendors (v0.10.0). If you are using vendors on a sim, right-click and select edit to check the contents and see if your script is v0.10.0. They should be v0.10.2.
You do not have to update them, but what has been noticed is that, with the v0.10.0 versions, if someone comes along and purchases all the items from its inventory, and then someone comes along and tries to buy that item again. At the same time, the vendor is empty, the vendor will reset, and you will not be able to load more of the same item into it again with the same price without having to load and do the setup all over again.
v0.10.2 fixed that issue. When everyone was sold out, you would not get an item menu, and all the owner or anyone else had to do was load/sell back into it for the item to show up again for sale.
If you find your vendor is being emptied and it is driving you insane to have to reload it with items and set the prices again, next time it does this, instead of reloading it, please package it up and send it to Temujin Calidius (attila.katana) for replacement.
IMPORTANT: Make sure you empty your vendors of all items and coins before picking them up and packaging them for upgrading. This will prevent the need to try to find your old vendor in the database to re-link it. Saving everyone loads of time.