what percentage of agented fiction sells?

I’ve seen people guess 25-30%. But it’s hard to put much stock in any figure, given that every book’s chances are different and depend on a host of ever-changing factors. I’ve seen pitches that made me think, “Oh yeah, that book will sell instantly,” and it did. Those are the books agents compete for (assuming pitch and pages match). But outside of those very obviously on-trend books, it’s hard to say. Even the acquisitions committee that decides to buy a book can’t say for certain that it will sell well.

Nobody really knows because that data isn't released by the agencies. I've seen 50% thrown out as a figure but no idea what it's based on.

Any numbers would be guesses, but at least the thing to keep in mind would be given the parameters you laid out, the agent wouldn't have taken on the manuscript if they didn't have an idea of how and where to sell it. That doesn't guarantee success, but the hypothetical competent agent isn't going to want to be investing time in something that won't sell over manuscripts that will.

It depends on all sorts of factors.

With debut fiction, the agents I've asked have mostly said that they sell about half of the new authors they take on. However, this doesn't mean that if you're taken on by an agent you have a 50% chance of being published, because so much depends on the book and how it's submitted. Every book is different. Some are so good that they get multiple offers of representation, and multiple offers of publication; others barely scrape their way onto an agent's lists, and might well struggle to go further.

Established authors who have a track record with their agent, and who are represented by a good agent, are more likely to get most of their books published--perhaps all, if their agent is proactive, discusses with them how best to proceed and what books are going to sell well, and so on.

Authors with one or two books published, and lacklustre sales, might find themselves unable to find a deal.