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1. I don't have my copy with me at the moment, but any coins, metal items, or magic items should be unaffected by the fire and can be collected as treasure.
Depends. Coins will likely survive, but could be difficult to find in the rubble. Soft metal objects - gold jewelry, copper items, pewter items, metal plated items - would possibly be fire damaged, and possibly reduced in value to their weight of metal. Most gem stones would probably be destroyed. Most magic items will likely be destroyed as well - potions will boil and explode, scrolls burn up, spellbooks incinerated, wooden hafted objects will become kindling, etc. Even things like magic armor or magic swords could be damaged to the point that they are not functional - leather fasteners would burn up, hilt wrappings could be destroyed, and so forth. While this might not completely destroy a magic item if the GM was generous, it might well require a quest to find someone that could repair the item (which would generally be someone that could have created it in the first place) - "Yes, this hilt wrapping is wyvern skin... very important to the proper functioning of the magic. Without it, the dagger is just a sharp but mundane instrument. I need a new square of properly tanned wyvern hide to get this working again."
I don't remember exactly what the building contains to find, but my sense is that most treasure low level PC's are likely to find will be gone except coins, which will be buried in the ruins and harder to find than ever. Certainly I'm going to break out my old 1e item saving throw tables for everything that isn't obviously combustible and make it pass a saving throw versus normal fire.
2. Why deny XP for destroying enemies with fire?
Except things that can't move, very few enemies will actually be destroyed by fire. For those that can't move, I'd deny XP because they never represented any threat - it would be like shooting animals placed in a pit. For those that can move - snakes and insects and the like - they'd bolt and lair up near the house, and very likely move back into the rubble once it stopped smoldering. Intelligent inhabitants would simply flee down into the caves.
3. From the Saltmarsh and Environs map, the Haunted House is almost 5 miles outside of town. Would destroying a haunted house ruin the PCs' reputation with many people?
Not initially. But the smugglers will still want people to stay away from the area. They probably will happily lay quite if the PC's burn the place down, but as soon as someone comes snooping around again, they'll need to make very clear that hauntings are still going on, and
that will harm the PC's reputation big time.
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