After allegedly insisting to Kelly Clarkson that Molly the singing bird wasn't for sale, Art of Play is singing a different tune ... the art installation is now on sale ... with a $15 million price tag.

This is the very bird at the center of Kelly's feud with shop co-owner Adam Rubin ... the piece sparked a big ol' he-said, she-said about whether or not Molly was ever actually for sale ... and now the Brooklyn shop is looking for a buyer with deep pockets.

Quick feud recap ... Adam insisted Molly was permanently attached to the wall and therefore not for sale. He claimed the artist who designed the bird would be willing to sell "Molly's twin" for $15K and said he offered Kelly a different bird automaton for $2,000 that she could bring home that day, but she left without taking a look.

Kelly, however, claims Adam's full of crap ... she says the store owners treated her like Julia Roberts' character in "Pretty Woman."

Adam ripped Kelly on Tuesday, and on Wednesday our photog stopped by the Brooklyn toy shop and discovered Molly was on sale ... for $15M.

The description reads, in part ... "The world's most famous singing bird automata!"

No word if Kelly's still interested.