Hello friend, you're talking about a merchant like a broker for example? I work on the world of bitcoin exchange, so we start by checking that hey are well known trustworthy brokers or dealers. I use FxStreetLive, in their website you can find information on Forex Broker reviews and about the markets and forex exchange if you want to do this too!
I would not dare to use the services of a broker recommended by nonames on forums. I advise everyone to read reviews on such companies in advance. I hope that you will not fall into the tricks of scammers. I also recommend you to read articles like What you need to know about trading currencies | ZeroHedge . It will be useful for both beginners and pros.