Like a lot of accounting standards the valuation of stock on hand is subject to some flexible rules. There is no single method that is universally applicable, and businesses usually decide on a method that suits the nature of their business, the nature of the market and the nature of the items themselves.
When bringing items in from overseas factors that you need to consider are when to apply the exchange rate for goods purchased in a foreign currency, and where to get the exchange rate from.
I recently moved to a new suburb and noticed an ad in the community news for a baby-sitting club. The scheme involves sitting other people’s kids in exchange for reciprocal services. The baby sitting club is a central broker and maintains records of who is entitled to what baby-sitting credits. In this case the medium of exchange is baby-sitting hours.
In a similar vein I remember seeing a TV story about an enterprising single mother who had managed to completely renovate her home by creating a system of exchange with a number of local tradesmen. She went one step further than the baby-sitting club and introduced an exchange rate. Depending on the skill of the tradesman she offered a different exchange of her time.