JG are the initials of Julius Gischard, the maker in Germany. The scale is atmospheric pressure read in inches of mercury. Lower pressure is indicative of poor weather conditions. Love it- nice find.

I know nothing about antiques or selling but several hundred dollars for a nice shiny new barometer $5-800+ of a different type. Similar to yours but more beat up are going for $35-50 on ebay.

Well I think its super cute and would make it a nice home and be happy to pay you $30 US. Not sure of a way to get it to you w/o divulging your personal info... ideas? Or could give me site it will be for sale at🤷🏼‍♀️