Wow. The sign is bad enough, but the attitude in that reply really stinks.
If the local gun community becomes aware of it, it's likely his sales will suffer.
If people call or write to complain [would you please post his email] it might happen faster & he'll change his mind.
We have a gun store here in the Milwaukee (MKE) area which (for at least 3 years) had been turning over info on ALL its customers (purchasers & range users) to the local PD, which then ran them for restrictions (felonies, DV, restraining orders). Of something like 25,000 there were 47 "referred to detectives for further investigation" and something like 3 of those who were actually prohibited people.
(For those reaching for a calculator, that's 0.012% - twelve one-thousandths of one percent.)
When this scheme was published in the local paper, the reaction of customers was swift, negative, very public, & overwhelming. Despite a couple days of initial digging in his heels & saying he'd done nothing wrong, the owner eventually said he'd stop.
(Whether people believe him or not is up to them.)
I don't think there will ever be an apology, from the store or police, but the public got the problem corrected.