I would say that the word 'but' has a generally natural negative connotation to it in most peoples minds so I think having it in a slogan is not a good thing.
Then there is the punctuation of it. If you add a coma after the priceless as in "priceless, but for sale" you can just start feeling the negative vibe you get. It might be priceless but if it's for sale the might be something wrong with it.
If you take out the coma and leave it as is then it just sounds like you're trying to pimp out your girlfriends backside.
If you wanted to go the priceless route something like "Even priceless can be bought" or "Putting a price on priceless" (though that one might have TM issues I believe) would have been a better way to go.
Just my thoughts
A branding mistake, a contradiction in a very childlike way, almost as if English is not the owners first language, he should do him/herself a favour and choose another one. It reminds me of a sign I once saw in Athens ' Genuine antiques, made on the premises '
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