Definitely Craigslist
 

CL, CL, CL all the way for buying bikes - it has completely supplanted the formerly $5B USD classifieds market, and it is the absolute #1 option for placing ads - everyone uses it.

As far as insurance - shop around! Progressive is a company that advertises heavily, but I've often found that they are pricey.

Try esurance.com, State Farm, and Allstate as relatively cheap providers. There are sites such as netquote.com and insureme.com that can get you quotes from multiple providers.

Always remember - The upside of the USA being so litigious is a proliferation of insurance options.

Note that different states have very different sales taxes (i.e. Oregon) which can save you a lot of $$$ when registering the bike at the DMV.

Also note that different states have different policies on same-day issuance of new titles - in California, they mail it to you (takes about a week); others issue the same day.

Every state has its own DMV website and can answer questions by phone.

Do a little research on the sites mentioned,I think you'll be pleasantly surprized at the cheapness in the U.S. When I look at classifieds in U.K. magazines the numbers seem comparable but the value of the pound doubles the price for me.
At a recent auction in Fla. I saw a low milage Buell Blast [2000 miles] sell for $1300. KLRs and DR 650s can be had for less than $3000. Older Jap street bikes are around for less than $1000 if you can clean carbs and free up brakes this can be a workable option.
That said I've always wanted to see the British Isles , but I am balked by the George Bush 50 cent dollar. Come to america we're on sale.