You don't have to be an art critic to figure out that the drawing on the right is not a real Ernie Bushmiller panel, but Peteykins of Princess Sparkle Pony's Photo Blog counts the ways that you can tell it's actually a tracing, which is something I have to admit I never thought much about. It's not a preliminary sketch, either. It's just a bad copy done with markers on an index card. Amazingly, someone apparently paid $100 for it on eBay.
First: don't buy a Picasso, Miró, or Dalí on eBay, OK? But most of Greco's fakes are more modest items: so-called "convention sketches" on small cards by cartoonists and animators. Even genuine examples of these types of drawings are usually minor items, done for fans on the fly over bustling tables crowded by onlookers, not the best circumstances for careful draftsmanship. That's how Greco gets away with it: he excuses the poor quality of the drawings by "admitting" that they are minor curiosities with, after all, affordable price tags. It's a pretty good scam!
Your best bet: Buy directly from the artist, which has all sorts of other benefits as well. Of course that won't work with Bushmiller, because he is dead, but if a seller is offering works by several artists that all look suspiciously similar, I'd give them a pass.
(Via Colleen Doran.)