The dogmatic answers you have gotten are very suspect for this simple reason: no one knows.
The issue isn't whether it's legal to sell one gun person to person but rather what makes you a gun dealer. 0bama can't enact law on his own but he most certainly CAN order the ATF to behave differently.
Before his executive action, it was commonly known, for instance, that one can not go to a gun show, buy a gun and then turn around and sell it same day. There is no law against this, but if the ATF were to hear about it, this crossed a guideline they used to determine if you were in the business of selling guns. The executive action seems to prod them to expand their guidelines as to what constitutes being a gun dealer.
Can you sell one gun now face to face? Two? A dozen? Beats me - no one that knows is saying.