a thread with potential!
It's easier if you know the former owner. A yard sale might be a different story.
Questions I'd want to ask:
1. Are there bits of colored gravel I will come to regret? Are there bits of crushed coral or other limey (sorry, mates!) stuff to raise my alkalinity? Is this stuff I would buy?
2. Is the substrate completely dry? How long since this was used?
3. Has copper sulfate ever been used in this tank? Copper precipitates, but comes loose again when the pH drops, stressing your Wood Shrimp etc.
4. Were there Malayan Trumpet Snails in the aquarium? I'd hunt for their telltale cornucopia shells anyway. Now, I like Melania myself, but the difference between marriage and Melania, is that Melania is forever!(drum kit: ba-da bum). You don't want to introduce any snails unintentionally.
5. Is my tank going to be planted, so that I'd be glad of the dried organic humus in this substrate? Or do I plan to wash everything out except the coarser gravel?
What kinds of questions would other people be asking?