No, the poison itself is untradable, whereas the fish can be attained from the Grand Exchange, Gourm' Implings or Fished North of Shilo Village, slightly North East of Harpie bug swarms.
The same place where you can buy and sell most things - the Grand Exchange or by an exchange with other players.
No, it's only in the main Runescape the Grand Exchange was made.
Grand Exchange
The first Runescape player to say g.e. for grand exchange was WEEDYMUN.
Antipoisons can be purchased at Jiminua's Jungle Store on Karamja. Or from the Grand exchange which is easier
grand exchange
Grand exchange
Grand exchange
At the Grand Exchange.
the grand exchange on runescape
There is no special "Grand Exchange account". You have your RuneScape account (just log in normally); you go to the Grand Exchange and start buying and selling things. First, you have to talk briefly to the guy (NPC) with the megaphone, south of the Grand Exchange. This is once only.
buy it on grand exchange