I don't even ask if the LFS has done any QT measures. I figure QT will be my responsibility ... and I implement 8 weeks rather than the usual 4 or 6 that I hear about. I added the additional 2 weeks after I lost almost everything 6 years ago from a combined velvet/ich outbreak (my fault for not quarantining properly) ... and damn, that combined outbreak hit hard
For me, if the LFS does some QT, that, in itself, would not raise the price I would pay. But, if the fish still looked in great condition after say 2 weeks (QT or not), I have more confidence in it surviving in my care, and have paid a slight premium for that assurance.