What is the ruling on buying and selling currency online through the mentioned websites?

Immediate possession is a condition when selling gold for gold or silver for silver, even if the types differ, based on the Prophet's (peace be upon him) saying: "Do not sell gold for gold, nor silver for silver, except like for like, and do not prefer one over the other. And do not sell something absent thereof for something present except hand to hand." So, if possession occurs by the amount entering the buyer's account in the same session, there is no harm.