I'm a U.S. citizen. I bought a used BMW F650 GS GD Dakar in England last Spring (2009). They were about the same price in the U.K. as in the U.S. so I saved about $1000 shipping each way (maybe you can find cheaper shipping...).

I used the address of a "friend of a friend" to register the bike in the U.K.. One potential problem (and benefit of buying in the U.K.) is insurance. I found a number of companies in the U.S. who would insure a bike registered in the U.S. and shipped to Europe but could find no U.S. company that would insure a bike purchased in Europe.

I sent numerous e-mails to companies in the U.K. and in other European countries inquiring about insurance. I told them the whole scenario: U.S. citizen buying a bike in the U.K.. Only a few were willing to sell me a policy and they were VERY expensive. I met an English gentleman who told me: "You're giving them too much information! Just tell them you want to buy some insurance!" Me: "What about my U.S. drivers license?". Him: "Tell them you just moved here and you haven't got a U.K. license yet." Worked perfectly. They asked how long I would be in the U.K.. I told them for at least 6 months, perhaps indefinitely. On the downside, the policy was only good for 60 days outside the U.K. and provided no coverage for most Eastern European countries. I bought "green card" insurance in Luxembourg (google: Arisa Assurances/very reasonable price) to take effect when the U.K. insurance became null.

If you have a E.U. passport, a lot of that is irrelevant. You'll have to do the math yourself to decide if it's worth it or not. Buying a bike in the U.K. was fairly straight-forward and hassle-free for me. It's now registered in Bulgaria (where insurance is really cheap and good throughout eastern Europe) but that is another story.... Good luck!