radioactive accident allows him to convert into mass of silicon particles, changing shape, expelling sand blasts, dispersing and reconstituting his mass, absorbing outside sand to increase mass, splitting into separate bodies controlled by aspects of his personality, etc.
Recruited as a reserve substitute in Avengers (1st series) #329.
Sandman experienced an unlikely redemption arc in the 80's and 90's. He started to abandon his life of crime following Amazing Spider-Man (1st series) #218 and Marvel Two-In-One #86. This led to a respectful friendship with his former foes Thing and Spider-Man, Avengers membership, and life as a rough but honest mercenary. However, his character growth was literally undone by the Wizard in Peter Parker, Spider-Man (2nd series) #12 using his ID Machine to revert Marko to an earlier persona.
Sandman had one of the shortest tenures in Avengers history. He was a reservist between Avengers #329-333. In Amazing Spider-Man (1st series) #349, Sandman attempted to act as a hero only to learn that, because of the technical restrictions of their current United Nations charter, the Avengers couldn't actually fight street crime as it was "too small" for the diplomatic superhuman task force. Sandman handed in his credentials after learning this.