In QLD you are allowed to sell the plates if you wish - lots of people seem to buy unique names then resell them for a fortune - yesterday at the Motorshow the PPQ stand had a large display of plates for sale - some as much as $50,000!!
But generally cashing in your plates is getting back the few months rego owed on the plates - if you don't have the car you don't need to pay the rego/road taxes etc - so handing in the plates is like a receipt fo rthe rego - you get back the pro-rata amount