Media buying and selling runs through marketplaces at every level: programmatic exchanges, direct inventory deals, podcast and newsletter sponsorship networks, and the rapidly growing market for creator placements.
Mediamarketplace.net names that plainly. It fits an ad exchange, a sponsorship platform, an inventory marketplace, a rights and licensing exchange, or a directory connecting advertisers with independent publishers. The independent publisher angle is the most interesting: newsletters, podcasts, and niche sites are collectively enormous and individually too small to sell efficiently on their own.
The .net reads as network, which is what a marketplace actually is, and the extension has a long history in advertising technology where nobody has ever cared about it.
Two common words, no hyphen, and the compound describes a business model rather than a product, which means the name survives whatever the platform ends up specialising in.
The name is listed for sale. Enquiries welcome.