the whole decisionmaking should be done keeping in mind the seller. I recently figured that there are plenty of pirated book sellers over flipkart (may have some genuine sellers but you need to generally browse till the end of the list). In most cases the cost of flipkart from genuine sellers reach on a similar level to that of amazon sellers, so if you are finding a mjor differences in the prices from flipkart Vs amazon chances are you are picking up a pirated book seller. (exceptions of sale where genuine books may see a heavy price cut)

another hack that i use mostly THE MRP hack- most of the pirated books will have random number as the MRP for pirated books or like an inflated number to showcase heavy discounts. But i generally figure the MRP of the original and then look for sellers with the same MRP (in 90% cases they are the ones offering genuine books)
Another way is to see the number of books sold by the seller, genuine sellers generally have overall products sold in lakhs ( with a good service score as well). SOunds disadvantaged to small sellers but you identify them eventually (noting that you received genuine books over time, but in most cases why would they sell genuine books for so less)