Your title says 500 free listings, but your post suggests it is a listing limit increase.  Are you aware there is a difference between the two? 

If it is a limit increase to 500 that means you are allowed to list more items, but it doesn't necessarily mean those listings are free, you will still only get the same number of free ones per month. 

If it is 500 free listings, then it's nice to receive them, but it won't do you much good if you are restricted to 80 sales a month, because regardless of how many freebies you have available, you won't be able to list any more than 80 in total.