I was just thinking about this...but don't customs at the airport make you fill out a form asking if you bought any merch when coming back to your country? They usually give a fine if you do not "claim" anything to pay the taxes for your purchases.
Yes, you have to provide proof of purchase for everything you have on you when you leave the country, so they know it's yours when you get back.
When you enter Brazil, you can bring anything personal (clothing, 1 cellphone, 1 camera, shoes) for free, plus $500 of anything else. You pay half the value of what exceeds that. So, if you bring a PS4 and 4 games, that's 640 dollars, so you'll pay 70 dollars in tax (total 640, minus 500 = 140, half of that is the final 70).
But that's if you go trough the tax route. At the customs, you have the option to get the declaring line, or the nothing-to-declare line. The former is what I said above, and the latter is a gamble. If you look suspect, are nervous or unlucky, they'll ask you to show your stuff. Then, instead of 50% of what exceeds $500, you have to pay 100%. If they don't pick you, you're tax free.
What people usually do is they go to the US with nothing but their clothes, money, ID and their cellphone's SIM Card. When they get there, they just buy everything they need, as it will be less than half the price of stuff here and tax free. That's my plan anyway.