see you auntie said:
It would be like one of the many conventions asking Hamill or any other Star Wars actor to appear for free when the organisers are making money off ticket sales.

Actors, of any movie or TV show, are not typically paid to show up at conventions (Creation might be an exception to this). They do usually charge money for a photo which they will gladly sign, but they generally show up for free. Hamill especially shows up at Comic-Con all the time because he's a total comic book fan. The ticket sales essentially pay for the rental of the building and not much else (at least with Comic-Con).