Well, here is a guide to collecting Tintin memorabilia (mostly books) on eBay.
As a short introduction, the 24 Adventures of Tintin are a comic book series written and drawn by the Belgian Georges Remi, commonly known as Herge. The books star young reporter Tintin, his little fox terrier Snowy, and a multitude of other characters, most notably old sea dog Captain Haddock, bumbling detectives Thomson and Thompson, and deaf genius Professor Calculus.
In the english language, all 24 of these books have been released. There has also been several activity books, special editions, and biographies released about the series and it's creator. In french, far, far more have been produced, with new ones released by Moulinsart (the merchandise part of the Herge Foundation) every few months. As collecting french books would be very costly due to the amount and incredibly difficult, this guide concentrates on collecting english books. That's not to say that a few french language books in your collection will do it any harm, of course they won't. In fact, some of these english books can be bought for far cheaper in french so if you speak the language why not give those a try. If you intend to buy foreign language books remember that it is far better to set your search options to search worldwide.
Of course, a complete set of the books is an excellant collection in itself, which will bring you hours of entertaining reading, over and over again, but only 22 of the books are currently available in paperback (all 24 are available in hardback) . I would definately suggest getting the full set before you try to buy other, rarer books, designed mainly for the collector.
Foreign language editions are always an interesting part in any set, and books in French, Dutch, German and Chinese can be bought usually for a small sum. Other languages, such as Thai and Bengali tend to go for a bit more. Again, setting search options to look worldwide is a good option.
First editions are generally readily available, particularly the "Tintin in the Congo" colour first edition, which was only released in 2006. Usually the older first editions go for around £20-£30. For US first editions, they go for more generally, as they were published by Golden Press books and have often deteriated due to the nature of the materials used to make the books. Prices for these really vary, particularly due to condition. The least you would probably pay for a golden press edition would be £20.
Three Film Books have been released. These are books not written by Herge but based on the Tintin films. The first two, "Tintin and the Golden Fleece" and "Tintin and the Blue Oranges", rarely go for under £50 and were only published in the 1960s. The french editions are far cheaper and are readily available on eBay France. The third book, "Tintin and the Lake of Sharks" is readily available in both paperback and hardback in english, having been published on numerous occasions.
Biographies and Companions are often a good resource for studying tintinology. "Tintin The Pocket Essentials" is a very cheap and useful guide, however is more of a referance book containing many dates of publications etc. rather than a good read. "Tintin the Complete Companion" by Michael Farr is a very informative book, filled with pictures, and particularly useful if you are interested in the sources Herge used to draw from. This is still readily available from under £10. "Tintin and the World of Herge" by Benoit Peeters is very informative too and has a section on biography as well. This goes for around £20 and often copies are ex-library for some reason. "Tintin and Herge Reporters" by Philip Goddin usually gooes for around £30+. "Tintin, Herge and his creation" by the late Harry Thompson is an excellant unofficial book, though it contains very few images. This goes for anywhere from £17 to £30+. Another interesting piece is found in english in the book "Drawn and Quarterly 4". This is a comic about the life of Herge entitled "The Adventures of Herge". Recently, several more books have been published to coincide with Herge's centenary, such as a set of books, each on a different character.
The Making of Tintin series is four books each containing one of the two part stories, such as one has "The Secret of the Unicorn" and "Red Rackham's Treasure". The first one, containing the afore mentioned stories, is the most common, usually reaching £20, while the others reach £40+.
Activity and Puzzle books. These are mostly for young children, but can still bulk up your collection. "The Tintin Games Book" is still readily available in both paperback and hardback for around £7. Recently, the "Tintin & Snowy Albums" have been released and can be bought for under £10.
Other books do exist, and these vary in price wildly. Notable pieces include Tintinologue and the Explorers on the Moon Pop-up book.