You shouldn't need to sell yourself to find joy, contentment, fulfillment; those who try are doomed to be miserable.
But, selling yourself is still important; even if your sense of self-worth and etc isn't tied into recognition, it is still something humans crave, and it has societal value. If you are doing anything of value to others, the only way it can achieve that value is by letting those who would value it know it exists. That's called "selling". Without it, what you've done has no value outside of yourself.
That's not to say (to your point) that something done just for yourself is without some sort of objective value, but it certainly has no value to society (by definition), and we are social creatures; we all have a desire to have at least some of our work be valued by others.
I think what the post you're replying to is getting at is that by framing things this way we overvalue things which others approve of, and undervalue things we enjoy doing for the sake of doing.
I think this is a very American thing to do. In most other cultures this is not true. Notably the OP is presumably British and my British grand parents most definitely were not "always selling" nor "putting their personality on display".
Since we are social creatures, it pretty much is. Social mores and niceties, for example.
Anyone is free to reject it. And that person is likely to live their life without their work being recognized, their ideas being appreciated, or their ingenuity to find the hands of those whom it would benefit.
I wonder how fulfilling that feels?
I'm not buying it.
Showing people that you care about them (and their interests) might be a/the purpose of life, and this is one example of that.
So, I'd argue we are always "selling" ourselves in one way or another, both unconsciously and consciously.