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).
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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
But in a quick look at eBay one finds plenty of too-good-to-be-true art auctions.
She had no reason to trust this stranger and his too-good-to-be-true offer.
As with all too-good-to-be-true mysteries, scientists have a simple explanation for this wall of wonder.