Can describe how big is the land, at where, what is the title of the land, is it a rural area or inside a city?

If it's a hot location under residential title, I think not hard to sell it.

Putting selling aside, although they already stayed there for generations, but they still consider as a trespasser if they do not have any documents that provide that they own the land.

Then if they don't own the land but got electricity, how about you go change the TNB name and cut it? They can't reactivate it back since they don't have the title of the land and they don't have any tenancy to change back the TNB name.

In law you cannot cut electricity of existing tenancy. But if it's no tenancy, you can free to do so.