Not only would certain trace elements be missing from the "reclaimed" salt, but you'd also get all the nasties (nitrates, phosphates, etc) back in the tank. Kinda defeats one of the purposes of a water change - to export nutrients out of the tank.
now that I am no longer buying lots of equipment I don't get free shipping. A 30+ pound of salt costs a lot in shipping!
Check out DrFosterSmith... you can get salt, sand, and other heavy items there with no additional shipping fees. Not free... but you're only paying $10 or so (standard shipping) for a 40 pound bucket of salt, delivered to your doorstep.