I don't want to rain on anyone's parade, but the chances of picking up a bargain on eBay when it comes to a genuine Hergé original are vanishingly small to non-existent. Not impossible, I grant you, but given the fact that eBay is currently carrying several pieces purporting to be genuine, but none of them standing up to scrutiny, I fear the possibility is unlikely.
Early Hergé art, especially if signed, is extremely valuable, and Ottokar
art when it comes up for sale had commanded hundreds of thousands of Euros. Simple head and shoulders sketches of Tintin and Snowy can get several hundred Euros with good provenance, and early examples more so. An unpublished or otherwise unknown piece, destined for, but not included in, one of his most popular works, signed, would increase exponentially in value and price, and be a star lot in any auction house sale.
Anyone savvy enough to recognize that it was valuable enough to sell, and knew enough to sell it as unpublished, would also surely know that eBay at a low price wasn't the way to sell it, when it could be sent to auction at an art sale and realize substantial sums; it's hard to reconcile one with the other.
As I said in an earlier post on a similar subject
: "airy disclaimers way down in the body of the text about a lack of certainty about whether or not they are genuine, and being sold as "in style of" fail to meet the high standards such transactions should take place under, and are unlikely to hold up in court should a buyer pursue the seller; they certainly ring all the alarm bells going about the legitimacy of the sale, and the seller's intent"
It's also notable the number of people who, when selling off their weakly-defended Hergé pieces, are also selling other equally questionable works by other modern 20th C. masters - it's not impossible, but seems odd that they might have uncovered a stash of sketches by Klee, Picasso, Mondrian and the like at the same time, known that they were saleable, but not bothered to see if a gallery, auction house or even a friendly local art teacher had an opinion on their worth.