The highest marginal combined tax rate in the U.S. is 51.8% (37+10.9+3.9, assuming an NYC address).

However, this is business income, not compensation, so it's taxed on a net basis, not a gross basis (even though it may still be included on his personal income tax return). This means his taxable income is the amount left after taking into account the retailer's fees, subcontractor costs, etc.

So, for example, if he made $100 selling games, $30 would go to the store. Assuming no expenses and overhead (since we have no data to come up with those numbers), the remaining $70 would be subject to tax. Assuming he lived in NYC, he would pay up to $36.26 in combined taxes (not taking into account the SALT deduction or the progressive tax rate calculation), for a post-tax net of at least $33.74. Assuming he lived in WA as other commenters note, he would pay up to $25.9 in federal taxes, for a post-tax net of no less than $44.1. (But note: Washington has an excise tax on businesses which is based on gross income...)