You put it on market, and wait for it to be bought by anotehr player.  You get more moeny that way, but you haev to wait till they are bought.  Also you can set the price to a percentage of the market price.  If you max out on price, someone can put cheaper untis ont he market, and theirs will be bough first.  Also if you put the price really high, you don't sell all your crystal/metal in one go, but only a fraction, due to the high price.  So if you put 100 metal on market, anotehr player might have to buy 400 metal for all your metal to get bought out.