Buy, sell or swap your precious metals. This section is for PM located outside of UK & Europe, US & Canada, Australia & New Zealand. Please indicate which country you are shipping from, and which you will ship to in your thread. If you only wish to sell to buyers within your own country, this is also fine to list here but please state this. Also can be used for International wanted requests.
Buy, sell or swap your non-precious metals related items. This section is for non-PM items located in any country. Please indicate which country you are shipping from in your thread. illegal/contraband items will be removed.
If a buyer wants a decent amount of metal - several tonnes, this would be done through the spot [cash] market. A buyer would go long XAG/USD or whatever currency was being used. This is transaction on the currency market; gold and silver are currencies. This locks in the price. These contracts are then presented through the broker for delivery.
COMEX is the exchange - the Commodities Exchange, part of the New York Mercantile Exchange [NYMEX]. Lots of futures contracts are traded through the COMEX - some are for gold and silver. This has become primarily a paper market. Rarely do longs demand delivery.
Silver mines might sell metal. It would depend on the mining company. Samsung for example came to an agreement with Avino Silver and Gold Mines ltd in 2015 for silver. You could buy on the Shanghai exchange in China and get delivery of silver.