transferring bitcoin among friends and taxes
They will most likely see any BTC credit in your account as "income" and you are taxable for it, unless you can prove it otherwise. This is a matter of when the IRS will start cracking down on these exchanges and request them to provide a 1099 for every user. I do not do trading via fiat because of this. The end-game is to acquire more Bitcoins, not fiat. 

When its time to sell your Bitcoin for fiat, you won't have to because everyone will be willing to take it. -- See this: http://imgur.com/gE8hDnY