If it is stolen (likely) it is worth 3-7 years.
If it is not stolen than $2000-$2500 if in good condition.
All of them have a serial number. How do you know if it is legal? Well you don't unless you bought it yourself as a LEO/ MIL. They simply are not available new on the civilian market. I have seen a couple slip over from LEOs legally into the market but it is a rare thing. I would regard any with deep suspicion.
If it does not have a serial number it is almost certainly stolen.