Welcome to the group!
When you trade in any firearm, expect to be lucky to get 50% of it's value if you sold it outright. SAR-1s without box or other stuff it came with are currently selling for $600 to $700 on the open market. Their value has gone up since they were banned in early 2004. Since a dealer is rightly looking to stay in business by buying low and selling higher, I would guess you will be offered what the 2003 selling price was, $300. You would be much better off trying to sell it on the open market.