Frank, et al:
I think one thing one must keep in mind is how "slippery" the value of a collectible like an old calculator is, especially when buyers having a wide range of knowledge bid on items they only see pictures of, described by sellers of questionable knowledge and motives. What I'm trying to say is: There's a lot of variation.
For example, I tracked all the Ebay HP-15C sales I could find (with simple searches) during the first four weeks of September, 2003. The result was 27 sales at a (rounded) average of US$ 208, WITH A STANDARD DEVIATION OF $55. In other words, the standard deviation was 25% of the average. This illustrates how hard it is to get a very accurate idea of what the units are selling for with informal monitoring of a few of the sales. (And of course I didn't try to differentiate by stated condition or accessories included.)
I would expect fluctuations in currency exchange rates to affect selling prices when international sales take place, but I think that other factors (including the large variation in selling prices) would make the effect of that rather small change in exchange rate (small compared to the big standard deviation) hard to notice.
Buying and selling prices on Internet auction are a very complicated issue, with many different variables involved. As I mentioned earlier, actual "value" is a mighty slippery commodity.
FWIW...
Larry