Assuming a bunch of economic actors across the world decide to conduct
business in a basket of commodities (say Gold and silver) - and call
each Unit of this basket "FooBar", what are the legalities of these
economic actors trading with one another in "FooBar" instead of a
This is legal, and indeed is done routinely in some sectors of the economy, either by denominating a transaction in a third-party currency (e.g. many Polish mortgages and international petroleum contracts are denominated in U.S. dollars even when no party to the transaction has U.S. ties), or by some indexing amounts to a commodity (e.g. gold or oil).
There are consequences to doing this, however. I can't speak to the German tax law implications of doing this, but in U.S. tax law, barter transactions have to be converted to U.S. dollars on both sides of the transaction before U.S. tax laws are applied to the transaction.
So, in the course of preparing a U.S. tax return for the parties to this business arrangement, a CPA filling out the tax return has to establish a conversion rate for Buffalo wings or FooBars to U.S. dollars and then has to complete the tax return on this basis.
Barter transactions are taxed under U.S. federal tax law as if the seller sold whatever was sold for U.S. dollars equal to the fair market value of what was sold, and as if the buyer sold whatever was sold for U.S. dollars equal to the fair market value of what was purchased. See, e.g., this IRS publication on the subject.
There are some narrow exceptions to the taxability of barter transactions, most notably for like-kind exchanges of investment real estate (governed by Internal Revenue Code Section 1031), for transactions between spouses, for transactions between entities disregarded for tax purposes (like single member LLCs) and their owners, and also, for example, for certain financial instruments like exchanges of stock certificates in the same company which are identical except that they have different certificate numbers. But none of those exceptions would apply to the situation described in the question.
There are tax regulations and other authoritative U.S. Treasury Department publications that provide detailed guidance on how a CPA is supposed to do this on an operationalized basis. I'm not personally sufficiently familiar with those regulations to describe them in any detail (because in domestic commerce, which is what most of my tax work involves, nobody does business in Buffalo wings), but I would know where to find them if I needed to do so, as would most CPAs and tax lawyers.
There are separate U.S. income tax rules for commodities transactions and foreign currency transactions. If you do business in Buffalo wings or FooBars or a cryptocurrency, rather than in a true foreign currency, the commodities transaction rules and barter transaction rules, rather than the foreign currency rules, apply to the transaction.