I would try to optimize it for one specific use case, not have a general API that does this. I think it's much easier to sell something more niche.

Find a job that requires reading long text. Let's say something in healthcare where they need to read a lot of journal articles. Now you're not a "summarization API" you're a way to reduce your time spent reading medical journals, time that could be better spent savings lives (your new tagline: "Less reading, more saving lives" -- half kidding). You can also optimize your tool to summarize journal articles which are written in a very specific way. When you sell, you can set yourself apart by being made especially for them.

I wouldn't sell to podcasters or anyone in media because they have no money (just look at how much writers are paid, look at media company valuations...).