Instead of imposing on the seller to get quotes, which I guess they are unlikely to do as it's much easier to sell, leave it in their garage and have it collected, why not get some quotes yourself?
A quick search on google of 'crated Shipping rates UK to US' gave me a page of companies such as this one:
http://www.anglopacific.co.uk/
No idea if it's a suitable one but they move baggage, cars, commercila items, exhibition equipment, etc.
Once you have an idea you'll know if it's viable and can then bid knowing you have a plan to pack and ship it
For fuel costs no doubt many with a Zed could even view and or collect such items for you as all roadsters can take a hardtop in the UK
Just a thought or two for you