you would need all shops should actually be 'clubs' otherwise legal tender laws would force you to sell your goods at the fiat dollar price. SO people become a member of a club probably for a hefty fee or perhaps a ritual to prevent people trying to 'rob' your store with fiat dollars. Once you are in the club you can price everything in dollars tied to the silver value. The captial gains taxes will motivate people to keep their money in the system-- if they try and cash out to fiat dollars there will be a 15% tax. The biggest problem is capital gains tax would also penalize the store owner when he tried to acquire inventory outside the system. The 15% capital gains tax may make prices uncompetitive. Barter would put him at risk of income tax audit (using the silver value to barter for goods would be risky). It could work for some limited kinds of products though. Dont know which. Although it may be a way for a person to be taxed at 15% of his income instead of the prevailing income tax rate. So it could work as a way pay capital gains instead of income tax.
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