You don't even need to use a strange medium like bottle caps to trade to be taxable. Straight out bartering product for product is also taxable. The only thing that is not taxable so far is food and other items that you produce yourself for your own consumption. They haven't yet taxed that although it wouldn't surprise me if they start. Of course they are going to try to prohibit you producing guns and if they do allow you to do it they'll tax those.