These Johann Hoff inkwells / inkpots are not rare - they do seem to turn up on eBay on a regular basis. See https://duckduckgo.com/?t=ffab&q=ebay+johann+hoff+inkwell&ia=web
Some people refer to such inkwells as "travel inks"
Value is in a wide range depending on condition. Follow the link above to find what some examples have sold for recently.
