Bananas are selling . . . . . . . . Rs. 250 per dozen?

'selling for' and 'selling at' are very similar, and in a casual conversation they can be easily used interchangeably, however they may refer to different situations. "the book was sold for $5" indicates a sale of one or more books. "the book was sold at $5" discusses the price of a book and doesn't mention the process of being sold. In the given question 'selling at' is the correct usage.