people already pay a taxes on their income and assets, and now you tell that we pay a tax when exchanging those same assets between us ?
Depending on your country/state you can be liable for sales tax on transactions (also called VAT, GST).
If the good you purchase then goes up in value, you can be taxed on that as well in some places. All of this is usually on top of income taxes. Yep, taxes suck so bad.