The secondhand market helps the firsthand market: people are more likely to buy something at the offered price if they know they can resell it on the secondhand market later.

Can you imagine what the automobile market would look like if you couldn't trade-in or sell your car and just had to trash it? New cars sales would plummet.

I think a used market exchange where someone had to buy each product is a different scale from one buyer who uploads infinite copies to the internet.

It's a tragedy of the commons. Not a commons of the product tho, of the ability to keep the creations flowing.