Ahaa! I see what you're talking about.
We use the idiom "sell off my assets" but I've never heard "dispose off my assets." I think perhaps because you may sell a few at a time, but to dispose of, or to liquidate assets refers to the whole package.
Well, that's not true either. I guess I really don't know why we don't say it. We don't say "liquidate off my assets either." (I don't think)