Perhaps the ZIP (free stuff) category is a mistake. A virtual category could be created by price=0 when posting an ad, and similarly whether looking in ALL for sale or in specific categories. People looking only for free stuff could check a box in the search options to see only free stuff (or could set max price = 0). Donors who don't know what category to select could use General while others would select a category as usual.
Note that ads without a price field entry would not default to 0 price. And they would continue to be omitted from price sort results.
However, the ZIP category has been around for a very long time, so expect some resistance.