Do I need to look or treat used substrate any differently (if buying a tank off of like craigslist that is used and it came with the substrate already)? How would one wash it but still have it aquarium/fish safe?
I've done this a few times, but I'd take a few personal steps in order to make sure that everything is cleaned out of it, such as;
Wash it with water and vinegar in order to get any dirt, debris, or other items out of the substrate.
Try to use some type of sifting method to remove things that may look like the substrate, but might be rocks, stale/old items, or metals/other things that could have come in if the tank was stored somewhere like a garage or outside for any period of time.