I maintain The Official Unofficial Groo List, which attemps to be a complete list of everywhere an original drawing of Groo (by Sergio) has appeared and anything that could be remotely considered a Groo item. It includes a list of all Publications with original Groo stories, reprints of those stories, International reprints, Groo merchandise and misc. items, appearances in MAD magazine, and original appearances in other publications and places. Some of these latter are very teeny tiny drawings. I also try to find all of these things for my Groo collection. It doesn't include original art or other one-of-a-kind items and it doesn't include reprints or preprints of panels used in ads.
Ruben, the very first person to subscribe to this list after Josh Jones created it, has tried to compile a complete list of everything that Sergio has ever done, while collecting it here and there as he can. He calls it the Sergiography. It's an open question as to which of us is more deranged.
No one has attempted to compile a list of everything that Mark has done and frankly, I think this would be harder to do because so much of what Mark does is essentially intangible. -Gary G.