How does it work if you sell at a KYC exchange and you pay taxes. But you say you bought it from someone IRL for 70k but you sold at 100k (hypothetically)

I've heard that unless you have proof, they'll treat it as if you bought it at 0 dollars? Is that true?
So you could end up paying more in taxes than you actually bought it for?
If so, how is that fair?

Why is bitcoin treated like this but buying gold or silver or metals, you can buy for like 20 bucks an ounce of silver, it shoots up to 50 and you sell a crap ton. They seem to be okay with paper receipts so is it really different with bitcoin? Idk... 

How about if someone gave you bitcoin as a gift? Do you have to pay taxes on that (In USA)

What ways can you legally avoid paying taxes aside from staying anonymous with it for life? Which means you can only sell btc for cash from ppl IRL which can be risky?