I’m not sure I can help you with your quest to own a copy of first edition paperbacks of Agatha Christie novels, but I wonder if you realise what a challenge you have set yourself. Christie wrote 66 detective novels and 14 short story collections and not forgetting the six romances under the name of Mary Westmacott. The cost of starting a collection of 80 books from scratch may be expensive and bear in mind the last book published was Curtain and that was in 1975, so first editions may well be over 42 years old and the Mysterious Affair at Styles 86 years old (the paperback edition was first published in 1932). Many of my own Christie paperbacks show signs of age and having been read.
You don’t state if you are looking for English versions of ones that have been translated into Indian – Christie’s novels have been translated into 103 languages.
My limited research shows that Tamil Nadu is a vast region in southern India, with a number of large towns, particularly Chennai. Reading an article by Zac O’Yeah about his Indian travels, he states that there are numerous bookstalls at the bus stands and railway platforms – particularly set up to catch the traveller’s eyes – where he purchased Christie novels. I can only suggest that you search the internet for the solution to your problem. In the UK, a popular place to find Christie novels (not necessarily first editions) are charity shops, where they will cost about £1 or perhaps £2. Another place to find individual books in the UK is Amazon, I have added to my own collection from this source. Then there is always ebay, however, most sellers are selling a collection of books and they may not be first editions and you might end up with duplicate copies. Bear in mind there may be postal costs involved in a purchase.
I hope this has offered a little help to your quest and wish you good luck.
Very interesting. I have been attempting to collect the first UK paperback edition of each of AC's crime books for over 20 years now. I am nearly there, but is complicated because there several different publishers involved and no definitive list of which editions I should be seeking. With time i have built up a pretty good list, but it is not 100% accurate. How are you getting on? Do you have list of the editions you are seeking?