Two media owners each put up space they are not selling this month and run each other's campaigns instead. A billboard company gets screens, a radio station gets bus sides, and neither invoice is ever raised. Empty inventory becomes reach.
Three steps, and the same proof requirement as a paid booking.
Live listings whose owners have said they will read a swap proposal. Propose one from your own desk — it takes a space of yours to make one.
Nobody has opened a space to swaps yet. Any seller can, from the listing wizard — it changes nothing about how the space sells for cash.
Post what you can spare and what you want back. Verified media owners see it immediately, and you keep the cash side of your business untouched.