If ask from current owner, surely the owner will paint the best picture, high revenue, low expenses.
Instead one should ask, why the seller wants to sell in the first place, or find the real cause of it
As if a stall/business is highly profitable, generally people won't sell it.
Best is to observe ownself to determine business flow and potential expenses or asking adjacent stall/business owner about situation there.
Questions to ask when buying a business?