Since your dealing in multiple currencies, the prices will never match exactly. The question is, who takes the hit when the exchange rate changes.
Personally, I think (that is in MY opinion, the guy sitting here typing this, not the moderator of ECF, not ECF, MY opinion, not my wife's opinion, not my cup of coffee's opinion, MINE) the price should be set based off the rate the day Imeo is paid (assuming payment comes before items). To use Bruce, since someone already has, as the example, Today Bruce buys from Imeo at 1e = $2 (yes, magical made up numbers), next week when they are for sale the rate is 1e = $2.5, but he still sells as if it was 1=2. Same would be true for the opposite, if you do it the other way (where prices should reflect current rates) then it's not fair to the reseller, besides the extra work he could end up selling at a loss just to keep it at 90e. This would mean that each time he gets new stock the price may be slightly different, but oh well.