I'm assuming from your note and previous posts that you are planning a trip to the US / Alaska, possibly also South America? If you are not leaving the bike in the US, you will not require a carnet.
From Court Fisher of BMWMOA on our Paperwork page:
"USA, Canada, Mexico and European countries do not require duty, Customs bond or Carnet to import personally-owned 'tourist' vehicles as long as they are re-exported within liberal timeframes, usually 6-12 months."
From our own and other's experience, we can also say that Carnet de passage is not required for Central America (vehicle details are stamped in the driver's passport, so the driver cannot leave the country without the bike).
In South America, a carnet will ease border crossings, but is not legally required and we know many travellers who have gone through without one.
If your agent is referring to a Spanish export tax of some sort, then I am very curious, as I would have thought that you only pay VAT once, which is when you buy the bike within the European Community.
I'd strongly suggest you check with another agent on this topic, and with your local automobile association, who would be the carnet issuers in any case. Please let us know what you find out.
Share the Dream at <A HREF="http://www.HorizonsUnlimited.com/index.shtml" TARGET=_blank>www.HorizonsUnlimited.com