All gun stores can buy legally aquired and properly serialized firearms, but there is an inherant risk due to not knowing the guns history, therefore most gun stores dont aquire guns.
I would propose gun vendor holders are able to run serial numbers of guns to verify ownership, similiar to the ability police have. To avoid abuse, put a cooldown on it, 1 per hour or x per day.
In real life, any gun store could run a guns serial number through a database to check if it was stolen or connected to a crime.