I've done some research here. To me, the opportunity is not in theme selling, but marketplace ownership. Theme forest charges a 20% commission on every sale, and it's basically an uncontested market place. If you can align the incentives correctly you can out compete them. Of course the obvious challenge is reaching critical mass, but with some creative thinking and early spending, it could be done. Find ways to incentivize people to submit a design to you and not themeforest, then send enough PPC traffic to them to convince them you're better than themeforest.

A lot of up front costs, but I definitely think the opportunity is there, especially as long as Themeforst charges 20% for a service with very few marginal costs.