Buy or sell?
Are these conversions done at buy or sell rates? The answer is neither. This is the explanation from Xenon Laboratories:
'Our rates are neither "buy" rates nor "sell" rates, as these are retail currency concepts. Our rates are, in fact, the large-volume money market rates upon which retail rates are based!
     'Our rates come from actual currency market trades. As a hypothetical example, suppose an international bank agreed to exchange 25,000,000 US Dollars to another bank in exchange for 52,500,000 Deutsche Marks. When this transaction occurs, it actually sets the new currency rate! (In this example, 2·1 DEM per USD.)
     'Is this a buy rate or a sell rate? As you can see, the question is meaningless in this context. The concept of "buy" and "sell" rates is only used for much smaller (retail level) transactions, where the currency merchant has to make a profit to stay in business. Retail rates are set differently (and arbitrarily) by the people who actually buy and sell currency.
     'Using these live rates, we calculate a time-weighted average for use in our services. Our rates always include the latest market currency rates, and are composed weighted towards market data in the last fifteen minutes. This mechanism, along with highly advanced logic for detecting mistakes in the feeds, ensures that our rates closely follow the market while minimizing the effect of anomalous trades.'