she buys up all the eyesores facing her shore, (lines 10-11)
Instead of being bothered by people not worthy of her, she buys their houses and then knocks them down, so she can keep her big piece of land and her ocean view.
we've lost our summer millionaire, (line 14)
So this is a town that draws really rich people, but maybe its year-round residents aren't quite as wealthy.
who seemed to leap from an L. L. Bean catalogue. His nine-knot yawl was auctioned off to lobstermen. (lines 15-17)
Here Lowell is making fun of this rich, picture-perfect New England sailor. He's also saying that the man left his boat behind (because he's probably so rich he doesn't need it) to lobstermen who are working-class people.