I doubt the player will be banned if they surrender the hacked weapons to an admin for disposal. Unless we develop a fool-proof anti-weaponhack system on SA:MP, then hacked weapons will get through from time to time.
People who are banned for purchasing hacked weapons, let alone from clearly obvious dealers such as a guy selling 2,500 M4 for $5,000, have earned their consequences through lack of common sense.
If anyone with at least half a brain saw someone selling suspicious-looking white powder in an alley near the police station, I doubt they would buy from him because they are looking for baby powder or flour.
Of course, this does not mean I support the bans on all innocents involved. I am merely pointing out that common sense (which in these days is becoming more of a superpower) can prevent plenty of these mishaps from occurring.