Actually Dorian, not "purchased." You can buy the ticket anywhere.

The fare is based on the country of departure of the first sector, the fare is quoted in that country's currency, and the ticket is sold in the currency of the country where it is issued.

So, a FRA originating RWSTAR1 will be quoted at 11000 DEM, which you can buy from a US ticket office for the USD equivalent, based on the prevailing ROE for the ticketing date.