well in theory they should know the book is priced in pounds, look at the exchange rate and price accordingly. likewise if i want a record that i think is worth £100, earlier this year i would pay $200, now its no more than $150.
what will be difficult, (ok, not as easy without working things out) in the future is looking back on past prices e.g. popsike; and knowing what the exchange rate was at the time. i had this same problem today - i was looknig at an item that sold last year on popsike for loads more $ than one last month...
It will cock up all the calculations in the fututre I tell ya!