Ok then, what if I have the product comparison module. When a buyer looks for HP Laptop 5000 from Seller A, but Seller B and Seller C also sell the same laptop; can the buyer be aware of this fact? Like a notification showing "this product also available from Seller B and Seller C, compare prices" kind of deal?
Yes that's what the Agile Price Comparison module does.
You must choose identify field from reference/upc/EAN13 to identify if the products are the same or not.
Seller must enter the reference/upc/ean13 code for when enter the listing so that the module will be find the products sold by other sellers and then it will be listed at as "Also sold by other sellers" at product details page.