If there was a fire in a residential property, it is likely that extensive renovation/repair/reconstruction followed, which would be a plus when selling the home. The fire should be disclosed to avoid legal issues later.
Yes you do. In the state of Virginia everything must be disclosed to the buyer at the point of sale. Especially a fire due to the circumstance surrounding the fire and the ramifications from the fire and the putting out of said fire.
The usual standard is "buyer beware." However, if the buyer has no reasonable means of discovering there was a fire (i.e. a reasonably diligent search of records) then you must disclose.
If the fire caused structural damage to the house or smoke damage, yes, it must be disclosed. If a claim was filed with insurance, you have to report it.
Yes, in selling a home, you must disclose any issue that could cause a house to be inhabitable. Otherwise, the buyer has a right to sue you for breach of implied warranty of habitability.