There's something known as 'upfronts' in the advertising industry. Advertisers buy ad-slots way in advance. Superbowl being a prime example. 

Primetime TV ad-slots sold this way should be NFTs. The creator would likely make more money and the advertiser gets more earned media.

great use-case I hadn’t heard before. 

would existing ad buyers or sellers want this level of transparency?

Depends on case by case. The prices are usually leaked/known anyway and usually there is hype around who is buying. This mechanic simply amplifies all of it.

The challenge is designing it so that the terms (brand safety, alignment) etc. are disaggregated from the token, almost like a pre-vet to be eligible to bid.