Does this mean that the book is being sold at a relative bargain, or is it a counterfeit "antique"? (paintings, for example, get faked all the time).
For the uninformed fan, the lack of printing dates in the books makes it a difficult business of identifying the age of a copy, especially on the spot in a shop. However assuming that the buyer knew what they were looking for - and all the important points seem to have been covered in this thread - I think it'd be pretty difficult to falsely age a Tintin book, unless someone happened to have in their possession some 1940s printing presses, inks, paper, colour separation plates and the like.
The most important thing to check for, as Harrock pointed out, is that it might be a facsimile printing. As far as I can see, the only reference to being a new book is on the rear of the title page, on the bottom left. If that part of the page is missing, it'd be worth making some inquiries as to the provenance of the book (which is highly important, not only for paintings). There are actually a few miniscule mistakes made in the inside of the book, where modern plates were used instead of the originals (some of the facsimiles accidently use the 'new' page numbering, whilst the original edition should have had handwritten figures).
Looking on eBay to get a rough idea of how much copies are selling on there for, there's one here
that started at 1€, and another here
, 90€ opening bid that gives a good range of photos, including the all-important back plate listing the titles that should appear on it and the blue lettering of the title page.
Hope that's of help !