I wouldn't worry about that. You can withdraw from your commissions earnings at any time (providing you've made at least $10, to avoid high PayPal/etc fees), and the system is much safer than risking new business with people you don't know. Lots of people have already successfully sold commissions and withdrawn from their savings; some have withdrawn three figure amounts already.

It's an entirely optional feature of the site, the only investment being a subscription, which offers plenty of other features as well. If MangaBullet was taking credit card details, a risk would be very prominent. But the site isn't, so as it stands, it's much safer than selling elsewhere.