an inverse argument...
Often times I have put "valuable" things in the free section and had zero hits, but then I put it for sale in the appropriate section for $20 and get all kinds of drool.
A great example... I have some used marine exhaust manifolds that are useless to me. I put them on CL for free. They went the whole 30 days without a single reply. Reposted... nothing. I put them in the Boat section for $15 and they were gone in a day.
First of all, someone who needs Penta Marine manifolds isn't going to be trolling the "free" section, and second, people tend to place more value on something for which they have to pay. I've watched free cars sit for days, but if you advertise a "free" car for $150 it goes in a heartbeat. Why?
Let's say you are looking for a free car so you search the free section of CL. You find a free 1977 chevy van with no motor, a hole in the roof, four flat tires, no title, and massive rust holes. You might get all excited and instantly reply that you'll be there in 20 minutes, but then you realize that its not worth filing for a title, buying tires and an engine... so you pass. But imagine if you place that ad for $200 in the cars section. Now, someone who is looking for a 70's disco van project will jump at the sale.
Here is a good story that demonstrates this. Before the internet years, my father often times put our castoff donation stuff in the back of his truck. When he finally got to town he would drop it off at Goodwill, but in the meantime he might trade it for something or just give it away free. One time we had an old TV that the picture was shrinking. He put it in the back of his truck with a sign that said, "free, needs repair, please take." It stayed there for over a month.
One day we were in town and he had an idea. He changed the sign to say, "TV for sale, $200." We walked into a truckstop diner and it had been stolen before we came out. If people think that something has value, they will value it.
This holds true for anything you put on CL. If you have a Miata worth $4000 and you put in on CL for $4000, you'll get a zillion replies right away from casual browsers who happen to see it on the first page. No one will actually follow through. When you finally sell it is when its on page 12 and someone who is serious about buying actually searches for a car that fits your description.
I reply to every buyer email I get, but I my excitement of selling is proportional to how long its been on the site.