1. Cop buys the kinds firearms regular people can buy using law enforcement discounts and then makes an otherwise legal private sale to profit from this. This violates the terms of these law enforcement discount programs. Also, federal law requires people who buy and sell firearms for profit have a federal firearms license. These firearms sometimes are used in crime.
2. In some jurisdictions like California and Massachusetts, law enforcement can buy certain handguns new that normal people cannot. This isn't about safety as these handguns are not capable of anything that the approved handguns are not. It's solely a measure to harm the market for handguns by restricting the kinds of handguns available and increasing their prices. Cops can legally sell these unapproved handguns to normal people. The problem here, again, is doing it for profit. These firearms sometimes are used in crime.
3. Cops buying "assault weapons" and then making illegal private sales to normal people who can't lawfully buy these weapons. This is nowhere near as common as the first two scenarios.
4. Cops playing games with post-1986 machinegun regulations so that federal firearms licensees who pay a special tax can acquire machineguns to rent to the public at ranges or just for their own personal fun. These firearms, which often are significantly more dangerous than the kinds of firearms normal people can lawfully acquire, have never been used in a crime.
Scenarios 1 and 2 have no public safety impact beyond private sales generally. Scenario 3 is only problematic from a public safety standpoint if you believe "assault weapons" are uniquely dangerous, which they aren't. Scenario 4 has not resulted in any violent crime.