You're not gonna get much for him. I see alot of big fish in classifieds for free because nobody will buy them, that's how I got my gar, most often it's pacus but occasionally an aro comes up.
Just take it back to the store that's no place for an aro....and many if not 90%+ of LFS will buy only 1/2 of retail so they'll buy it for $40 but sell it for $80 and as said no one is going to buy a big aro because they'll feel like they can get a smaller one and grow it out for cheaper as I know IME I like to get young fish and watch them grow big so I can look back and say remember when.....BTW, bring the fire extinguisher you're going to need it. many including me don't look kindly to buy little sell big, rinse repeaters.....