Unlimited free listings would destroy ebay. The place would be full of rubbish that no-one wants going round and round for years on end. Buyers would not be able to find things amongst the junk and sellers of quality items would see their listings lost in a see of valueless carp. 

When sellers have to pay for listings or only get limited free listings they make sure the items they list have some value and some chance of being sold.

It also costs money to service all of the listings both from a technological and human resources point of view. Who pays to keep all of those valueless listings running round and round ? The sellers of quality items that are worth something and actually sell of course.