Being selfish to some extent isn't a good a thing. A selfish person cares only for himself at all times.

If you're not selfish people will use you

I think in this aspect I wouldn't really say the person is selfish, rather one should try to stand their ground and know when to say yes or no to people when the need arises.
It doesn't mean you don't care for others; selfless

Not being selfish means Easy target

This is exactly what happens when people see you as a soft person.
There are people that likes using others simply because they are selfless