Actually for TV and radio there are several companies doing remnant ad sales which are fairly affordable and can be spun up pretty quickly. The hard part of TV ads on the fly is you need to have some production quality which can add expense and time. Not so much for radio. With billboards, I do know there are remnant sales there, but again you have a lead time to produce the banner that is attached to the billboard.

One company that comes to mind off the top of my head in the radio/TV space is Bid4Spots.com. Basically like Google AdWords for TV. There was talk at one time of Google offering a similar product, but I don't know what ever happened to it. Finally, there was another company that had an entire ad wizard for TV ads. Essentially you could pick choose music, or do a voice over, add some text, and have a generic video playing in the background. You could essentially create an entire ad and have it on TV in a day. I can't remember the name of the company though.

Wow, I didnt know this things actually existed.

On the hindsight, we will need bigger players. Because the physical Ad networks in different countries are so wildly different that it'll need to approach them one-by-one like Uber. Probably, they wont be able to buy in bulk just at first, instead will use some local 3rd-party providers.

Also, Bid4Spots.com seems to be radio-only. Did it support TV ads too earlier?

I thought it supported TV buying as well, but could be wrong.

ViddyAd.com is the name of the online TV ad builder I was trying to remember.

In any broadcast medium the broadcaster is expected to conform to community standards so somebody will have to watch the ads and do some editorial selection.

As for production values, look at the ads run by local car dealers, furniture stores and such. Images with text and voi eovers play well. If all else fails use the "Ken burns effect"

A person with a month or so of training and maybe $5k of gear could produce a few ads for a customer in a day and you have to hook up that person to the customer.

Yes, I understand that it probably wont be possible to run instantly. But manual approval should be possible within 24 hours, allowing for next-day ADs.

As for local Ads, in my country India those are run only in "local cable channels" which is basically some extra channels inserted by local cable providers (in US, I think(?) there are only nationwide cable providers). There is little scope for such deals in mainstream channels.

We have local channels put together by the cable company (and the phone company in some places), too. They mainly have video of community events, like town council meetings or school sporting events. There are a few that have tickers and scrolling panels with various topics and combinations of topics (general news, financial news, community news, weather, sports scores, stock ticker, etc.).