This may have to do with both regulatory approvals and CoO tariffs since the 110V (120V) version is generally intended for the USA & Canadian markets which require different safety and EMC certifications/approvals. There are also warranty differences, there may be no warranty unless you return the amp to the EU.
Also, any country with VAT may show pricing outside of those regions without the ~20% VAT. Add 20% to $500 and that's another $100 to the $500. Then, if there's CoO tariffs, that's another 20% and now you are at $700. These are independent of who is selling the product. Then there's the gamble of what the import customs duty, brokerage fees and other charges and this may end up right about the difference that you posted... except that you would end up with a correct 120V unit. No way to know until it's all over.
Sounds like you tried to save some money but may end up not saving as much as you hoped