I wouldn't say I'm a new player but I have never bothered to sell anything on the GTM before. For some reason (which I've noticed since day one) I cannot sell anything on the GTM. It says to "drag item here" but when I do nothing changes. I don't understand what I am doing wrong and advice on this would be greatly appreciated!!

Is the item you are trying to list on the GTN "Bound" to you, by chance?

As Tiberius indicated, some items cannot be sold on the GTN (items that are Bound to you would be of no use to anyone else). Though, when you try to sell something like that you should have gotten a message about it.

 

Also, I vaguely recall having problems with the "drag item here" function in the past (and not just in the GTN). So you could try to do what I do: Open the Sell tab in the GTN and then just right-click on the item you want to sell in your Inventory.