If the business is being sold through a broker or if you enlist the services of one, a good broker will usually stress the importance of providing financial data to ensure the deal goes through. You will probably have to assure the owner that you will not contact anyone without prior approval, including suppliers or employees, for additional information about the business. But it makes sense for you to ask to see relevant documentation regarding the operation of the business up for sale. There are variations in the kinds of information that you will need, along with industry variations. You may have to vary your checklist according to the particular type of business you are considering.
Some major corporations require checklists several pages long.
You should know the status of inventory, what's on hand at present, and what was on hand at the end of the last fiscal year and the one preceding that. You should also have the inventory appraised. After all, this is a hard asset and you need to know what dollar value to assign it. Also, check the inventory for salability. If you feel it is not in line with what you would like to sell, or if it is not compatible with your target market, then by all means bring those points up in negotiations.