I suspect you'll get less nett from the sale if you get it repaired commercially, parts for a boiler fix on a Silvia run ~$140 retail (IIRC), I'd be extremely surprised if you got out of a commerical repair for less than $200, finger in the air guestimate ~$250.
I doubt you'd get $250 extra selling it working vs not, particularly given your description of the cosmetic condition of it.
If you're interested in keeping it and you're somewhat DIY minded the boiler swap is pretty straight forward (search the forum, I posted some instructions for another user maybe 6 months back, there are also some YouTube videos around for it)
EDIT: Hmmm, just re-read your message, it heats but it trips your RCD when you turn it to steam? Is it OK if you just leave it sit there (turned on but not to Steam)? If so would tend to suggest that the boiler is probably OK, my suspicion would be the Steam thermostat or the wiring thereof, though I can't think of a fault mode there that might cause it to trip (particularly since IIRC the current for the element routes via the steam thermostat normally and the steam switch just bypasses the "brew" 'stat, which would imply the fault occurs when the steam 'stat opens, which would be rather weird)...
I suppose it's possible there's a split in your element which is opening up and allowing ingress of water as the element expands, but only does so at the higher temperatures of the steam function, very unusual failure mode though, try leaving it turned on for a long time and see if it trips, I expect if there's a crack in your element I'd expect the water to eventually get in there even at the lower temperature.