You need to send a private message to the seller telling him what you want to say.
These PM's are totally private and no one can read them except for the people involved with the message. That allows you and the seller to give private information without worry.
To send a PM, called a "Conversation" on this forum, put your cursor over the receivers name. That will open a little window. Select "Start conversation".
Fill in the title, type the information you want to type and then poke the "Start Conversation" button at the bottom to send it.
Look at the upper right hand area of your screen. Do you see the little envelope icon? That shows if you received a private message. If you did, a number will appear in the envelope. Click on the envelope and then the message to open it.
As your new here, it's a good idea to check your settings to see if you are going to get the PM's sent to you.
To do that, Click on your name in the upper right hand area of the screen. That is where you can change your settings.
Select, "Privacy". Then scroll to the bottom and look at the line that says, "Allow users to....." then the line that says, "Start conversations with you". It should be set to read, "members only". If it doesn't say that, change it so it does.
Be sure to hit the "Save" button at the bottom of the screen. If you do poke it, your done. If you don't poke it, the change you made won't take effect.