Schweickart's system, however, allows means of production to be bought on a market. This means that individuals could accumulate some money and acquire means of production of their own. so people can in fact start their own private businesses in his scheme. To prevent this from rebuilding capitalism, Schweickart has to rely on the state.
This just isn't allowed in the system. You are only allowed to acquire that which you can productively utilise yourself. If you buy more machinery than you can work with yourself, there is no point. To make it worth the cost, you would have to employ wage labour. But the wage labourers would have every interest in reporting you, collectivizing the equipment and getting rid of you as a boss. It need not be a case of the state intervening. It could be a case of the commune's members not recognising the right of the would be capitalist to the machinery he claims. If the commune says to the workers employed there, "feel free to take over that factory, we won't stop you" what will the boss be able to do about it?
In sum, there would be no incentive for anyone to acquire means of production that did not help them carry out self-employed work. The very act that makes this accumulation desirable and useful (the hiring of wage workers) is the act which enables this infringement of property norms to be detected (since any wage workers approached or employed by the would be capitalist have every interest in "dobbing him in").
One way in which the social investment process would be completely circumvented would be for firms to not pay their surplus as distributed income to worker-members but to reinvest in means of production to enhance their power in the market. In fact market competition would tend to force them to do just this. Groups could also lend their savings to others -- and we could see how a private banking system could emerge from this. Firms losing out in the competitive war might band together in a politial alliance to get the capital tax reduced, to enable them to keep their profits and expand further. Profits are a weapon in a competitive war. Thus any social control over means of production diminishes.
Schweickart rules out plowback investment of this sort. A depreciation fund is supposed to be kept to keep capital value of a firm constant, but new investment must come from the communal investment fund. One would imagine that firms accounts would be open to public inspection, and would be audited just like corporation accounts are.