If the gun is well cared for and essentially "like new", you could get upwards of $400 for the gun+mags in a private sale. New they go for 425-450. The accessories won't command the same resale price, but you could probably get pretty close to $500 for an all-inclusive deal. Again, if it's a private sale and if the condition warrants that price.
I'd list it at $500, and consider offers north of $450.