You can do /itemdb with the item in your hand and it will give you a list of short names that you can use.
Type [Trade]$<amount>825 exp:<Amount you have> *EDIT* That's if you're selling 30 levels at a time, which is recommended
Here you go. The data value (dec) is the value you can put into the trade sign. You can also do "/itemdb" with the item in your hand.