I can't find a reference on the Guiness World Records web site, but this news article from 2010 says:
And if you were wondering, according to the Guinness Book of World
Records, The World’s Longest Yard Sale” runs along a 630-mile stretch
of U.S. Highway 127 from Gadsden, Ala. to Covington, Ky.
Here is some more evidence pointing to the Guiness World Record in 2004:
Contrary to the hopes of some participants, this sale won't even come
close to breaking the Guinness World Record for longest yard sale.
"No, no," Weiler said. "There's a 450-mile yard sale on Highway 27
that runs through four states in the South - that's where I got the
idea for this one."