If you want to buy diamonds, press control-f and type "buy diamond" (with no 's' on the end, as the item name is "diamond", not "diamonds") and your web browser will take you straight to the information about buying diamonds. If you want to sell diamonds, just type "sell diamond" instead.
Some items have a code after them like (1A2B3) which indicates that they're not simply that item, but have extra data, like enchantments or branding or something. For example, a lot of people use branded paper as a currency and so if you see "paper (1A2B3)" then it's not just paper, it's someone's currency. You can then search "buy paper (1A2B3)" to figure out what you can trade to get their currency.
Where to buy :