Vendor mechanics?

[Aalderon Crystafire]

Is there any data on how non-'stock' vendor inventories work?

That means items in a vendors inventory that are not infinite or part of their original stock - only items sold to them by players.

Specifically, how they move around.

e.g. A merchant is killed and its inventory reset to stock. I sell a single rat whisker to it which now becomes its first non-stock item. Will this rat whisker REMAIN in visible no matter how much new trash is sold to the merchant, as long as nobody buys it?