G23, Gen 3 used is probably worth realistically around $425 or so give or take. A Remington 870 WingMaster used is probably about $300, maybe $325. We're talking WINGMASTER and NOT Express here, right? An Express is probably a $200-$225 buck gun at best mostly because of the finish. The WingMaster has a much nicer finish and stock then an Express.
If they want to deal I'd say your G23 for their Remington 870 and $100 cash or I'd not take the deal.