This is about collecting original art by comic book and comic strip artists. There's a lot of it offered on eBay and for the purposes of this warning, let's divide it into two categories and I'll use Charles Schulz and Jack Kirby as examples. It is also true with other artists.
There is Published Work. This would be the original art to a Peanuts strip by Schulz that was published in newspapers around the world or it would be a page of Fantastic Four or some other feature that was drawn by Kirby that was drawn for an actual comic book. I would say that at least 95% of these that are offered are genuine. Once in a while, you see a photostat passed off as an actual original. Sometimes — and this is true of Kirby and other artists who did not ink their own work — an artist will trace an old Kirby page and ink it himself. These are usually identified as "re-creations" but sometimes they're not or they get mistaken for the original versions.
Then there is Unpublished Work. This would be a sketch Schulz did for someone of Snoopy or a drawing that Kirby did of Captain America…or it could be of any of their characters.
In this second category, if the drawing is signed to someone (i.e., "To my friend, Mike…"), there is about a 30% chance the drawing was actually done by the alleged artist. If it isn't personalized, I would say that number drops down to around 8%.
And if the seller has lots of these — lots of sketches done by different popular artists — that percentage drops down to about 2%…maybe lower if Schulz spelled his name "Schultz." And yes, I've even seen that.