too good to be true

So excellent that it defies belief, as in She loves all her in-laws? That's too good to be true. This term expresses the skeptical view that something so seemingly fine must have something wrong with it. The term was part of the title of Thomas Lupton's Sivquila; Too Good to be True (1580).

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.

Examples from the Web for too good to be true

Contemporary Examples of too good to be true

But in a quick look at eBay one finds plenty of too-good-to-be-true art auctions.
Why eBay Is an Art Forger’s Paradise
Lizzie Crocker
August 19, 2014
She had no reason to trust this stranger and his too-good-to-be-true offer.
How An Israeli Stranger Helped An Ethiopian Girl Escape Prostitution
Itay Hod
February 9, 2014
As with all too-good-to-be-true mysteries, scientists have a simple explanation for this wall of wonder.
294 Dinosaurs Once Walked on This Wall in Bolivia
Nina Strochlic
October 24, 2013