There's no way to bind keys to text using the GW interface. But you can Copy + Paste from a text document if you enjoy spamming long messages or special characters in Lion's Arch.
Ah, I just wanted it to say I am selling a item are running ect.
If that's all you want to do, then type what you want to say. To repeat a sentence press "Enter" then use Up Arrow to scroll back up through things you typed previously.