I have re-use sand many times, I always wash it really well before storage, then dry it out. that said you will experience a burst of ammonia at start up because that bacteria on and in the sand dies and is left as dead bio-film on the sand, plus any phosphates will remain in the sand, and lots of dead organisms wont be removed by washing. you tend to have a higher algae production start to your cycle when reusing sand. but on the bright side you don't need to add an ammonia source. If it's a cost issue you can absolutely do it but be prepared for more initial work on the new set up.
Most people will recommend new sand and it's not a bad idea, but if you are prepared for it there's not really many reasons not to re use sand.
I would not use your sand for fresh water unless you ae doing a cichlid tank.
