eBay has long been a place where original comic art is sold, some of it even real. Yes, some of what's offered there is counterfeit and I'm often amazed at how bad some of the forgeries are. I haven't searched there today for "Charles Schulz drawing" but that usually yields a few that are almost certainly real plus a few that a blind aborigine could identify as forgeries. There was a forgery up there one time that was signed "Schultz"…and I believe that like many a fake drawing, its authenticity was confirmed by some agency that sounded like they knew what they were doing.
At present, sellers seem be offering an unusually high number of "Jack Kirby drawings" that Jack had nothing to do with. It's shocking to think that there might be someone who loves Jack's work enough to pay megabucks for a sketch…but can't tell that the sketch is not only a tracing but a really bad tracing.
I refuse to get involved in authenticating drawings, especially when requested to view a small scan instead of the original piece. There's no upside for me in doing that and too many people want to kill the messenger when told that the drawing they purchased — with money that might otherwise have put braces on their kids' teeth — is not what it was made out to be. But use some common sense, people. Go to eBay and if you can't spot some obvious fakes, maybe you're not qualified to be sure that the one you want to purchase is real.