In paintball, this is standard practice. However, field owners usually charge ludicrous markups, often for inferior quality products that get chopped in the gun. They make a killing on ammo sales - usually far more than their actual field fees.
As long as you're selling quality BBs (like Bastards or equivalent quality) and charging standard Bastard rates, I don't really see how anyone could legitimately have an issue with it. As long as you're not selling the cheapest BBs you can find at stupidly inflated prices, I doubt anyone would have an issue.
For Bio Bastards, I have no personal experience with them, so I don't know how good they are. But I suspect since they are Bastards, they're of the highest quality possible, so I wouldn't have any issues loading them into my guns.