if i am sell something can i ask for some diamond? Will i be able to have diamonds if i am not buying from real cash money?

Yes, you can sell things for diamonds. You just have to use the currency exchange to get them.

Say if you're selling someone a pet for 1000 diamonds, make sure the agreement is done via pm or forum post or somewhere on Sylestia. (Never make a deal off site.) You would set up the pet for that user selling to their specific user ID for an agreed upon amount. If the purchase is in pure diamonds, the pet would be set for 1 gold. You would then post a currency exchange request, selling 1 gold for 1000 diamonds. (The seller sets up the diamond exchange request because no one but the buyer would buy 1000 diamonds for that 1 gold, but a lot of people would snap up a 1000 diamond exchange for the cost of 1 gold if the buyer placed it, so there's no guarantee that it would be picked up the the correct player.)

You can also use the currency exchange to buy diamonds with gold, buy gold with diamonds or sell gold for diamonds or diamonds for gold from other players.