As an example, the mining company 'Original sixteen to one mining company' has the stock symbol OSTO but the shares trade on a company run market where people have to contact the company to buy or sell the shares but the results seem to get reported into the online systems. While the company still operates, gives tours of the mine and puts itself out there as one of the oldest mines in the US, it is basically mothballed for all practical purposes. (Although if there is a large rise in the price of gold then maybe they may begin some operations again.) There is also an annual report that people can get if they write to the company. That company is one example of a company trading in the grey market.