No he means that if you sell them, you have to pay the due taxes. Zynga and Valve supposedly pay their taxes. The difference is that in this case it's the individuals who make a profit and the IRS (or other tax agencies) would make such a "virtual goods for real money public marketplace" more complicated.
I believe he means if you manage to make enough money with the auction house, the IRS will be curious if you're reporting this as income and paying taxes etc.