There was a time when agorism was widespread: Prohibition. And the outcome suggests more problems for agorist theory.
The prohibition of alcohol created a perfect situation for black markets, an they obligingly appeared. The result was not predicted by Konkin's agorist theory-- instead of an ever-expanding black market leading to the end of the state, prohibition led to a huge increase in organized crime, which eventually prompted the end of prohibition.
Most agorists would predict that security on black markets would be provided by competing security companies-- basically, a black-market microcosm of full-scale market anarchism. What went wrong?
The problem, predictably, lies with the government. Black market entrepreneurs, in order to protect their businesses from government interference, were forced to bribe politicians-- rent-seeking. But of course, the rent-seeking didn't end there. Black market businesses began using politicians for other purposes-- paying them to bust up their competition while leaving the business with a territorial monopoly on its product. Thus, government corruption turns black market security production into black market governments.
This is undeniably a major problem for agorists. Not only because it creates governments, but because it creates the political will to remove the offending laws in order to get rid of the mafia. When the political scenario favors agorism, it gets altered. Large-scale agorism is self-defeating.
I had a great source for all of this history, but I can't find it right now. (Damnit!)