Yep, it's totally legal at least in the US, as long as you pay taxes on it. If you don't, that's a problem in the eyes of the IRS but the buying of it from a person in S. America (or anywhere else) is legal. Bitcoin isn't illegal in the US as you probably know, and in fact there are US-based exchanges like Coinbase that are major dealers.
Thanks for reply man. But let me see if I understand what you mean.
Once I sell my bitcoin(bought in S. America) to a Localbitcoins User in USA.. I should pay taxes for it? Or you just pay taxes in the act of PURCHASE, not the act of SELLING?
How would one go about paying these taxes? Talk to an accountant?
Thanks again.