What ever you do don't trade it in to a gun shop; you'll never get any where near what it's worth in trade value.
If your SGL 31 is Arsenal marked and not FIME it'll likely have a little more market value. Also, all the big retailers (buds, atlantic, jet guns, ect...) are out of SGL31s at the moment and don't expect any in until January... so that bodes well for you. Anywhere between $750-850 is reasonable in today's market.
I'm not sure when you bought your rifle and mags but surplus polymer 5.45 mags are going for $20 and up these days so you can sell those for good money depending on how many and what type you have.