Oh. This isn't that hard of a question. There is no rule saying you cannot have a 0 barter holding. It just means barter isn't received. Just as any other option that could trigger on a surplus may be there or not. There is no listed rule saying 1 barter is the minimum, and really there isn't a good reason to enforce a minimum. I'd suggest that it's probably a typo, but even if you do want a minimum (or don't), it doesn't matter so long as you're consistent about it.
Wealth is subjective. In this instance, the player clearly put eager recruiting over a positive barter value. Just because you aren't getting barter, doesn't mean you aren't getting wealth. Look at the people and the people they're bringing into the fold as a wealth for the PC. Make their presence helpful and valuable. They're a growing community full of potential/promise etc even if they're not rich enough to make a huge splash.
If this is a real example, then I would definitely start asking questions about the settlement. How are they recruiting, what is this holding? What makes it count as wealth to the player? Ask them. Is it in good deeds? Perhaps they're educating the people that arrive as a way to secure recruitment. Play to find out.
::Edit::
Now that I think about it, I had an example o this in one of my games. Basically due to all of the players ending up with gangs, followers, holdings, and each of them fighting with each other, fighting with neighboring settlements, causing chaotic wars and basically accruing massive fatalities over many many conflicts... They started to actually run out of people to fight for them. I didn't make it easy to refresh the gang size once they lost people. Haha. So one of the players picked up a holding that's entire purpose was to draw people in as a place of refuge, a place they could bring their families, a place they would then and thereafter be willing to defend. Thus, they started drawing in people and then recruiting from them into their gang. It was a pretty sweet move, because those people helped his gangs "endurance" in the power struggle at large, and in the end it amounted to a major victory over another PC that was much better off at the start of the story.
His wealth was: "The Moral High-ground" in this instance.