Hey there! When users first register, they are added to a 'Newly Registered Users' group which has limited posting and messaging abilities. This is to prevent spammers from posting adverts or inappropriate material in the forum.
To become a full member, you need to make a post in the forum and wait for it to be approved by a moderator or admin. Once it has been approved you will become a regular member and will be able to PM and send trade messages. Future posts won't require post approval. Since you were able to make this post, you should be able to send trade messages now. :)