What if this property is moved oversea and never exchanged at home country? If people avoid centralized exchanges where all the bitcoins can be traced to person, then there is no way to properly identify the ownership of each coin
Assuming the property is held by somebody in the US then they are subject to capital gains tax regardless of where the exchange is done.
Are there ways to hide income? Sure. But now you're probably also guilty of tax evasion and maybe money laundering. Does it matter if you don't get caught? I guess that's for you to decide.
The IRS depends on taxpayers being honest. Many aren't honest. Most probably don't get caught. Some do.