What does it mean to you to "sell yoursef short"?

It means that you aren’t properly valuing yourself. I sold myself short in a friendship where I let the other person act erratically and unpleasantly towards me.

If you undervalue your skills to someone else, you sell yourself short
I know as much, or more, about a certain piece of equipment at work as anyone, but I am always hesitant to give someone the answer to their problem with it, because I lack the confidence I should have.

It’s treating yourself and your feelings, thoughts and ideas as if they don’t matter, so they aren’t acted upon. It stems from a lack of self-worth.

I thought that I was a terrible writer, that my imagination was lame, that I wasn’t funny, and certainly not as funny as my heroes in Monty Python, Mr. Show and Kids in the Hall. Oh, and I couldn’t sing, was a lousy dresser, was fat and unattractive, so why bother putting myself out there at all? “They” were all just going to laugh at me. “What the hell is dis shite? Who da F are you? Get da F outta heah!”

Except all of this is untrue.

The world need your talents and skills. It does. And when you sell yourself short and hide them, you’re depriving everyone else as well as choking yourself off. That was a hard lesson or me to learn, that I’m not supposed to hide. Put all of yourself out there. That’s part of the remit for being alive, I think, as a human being.

How can you know if you are selling yourself short or if you have an accurate view of yourself? It only becomes apparent when you have a different view of yourself than other people have.

Some people think they are much better than anyone else thinks. Those people are really annoying, although they are occasionally funny. Other people think they are worse than what the general response to them is.

Do those who sell themselves short deprive everyone else as @aprilsimnel suggests? That suggests that we have some kind of objective duty to others and that we owe others something. It’s a commonly held point of view. I’m not sure I agree.

I think it’s kind of the other way around. If people want something I have, then they should ask for it. It’s not up to me to try and push myself on them. My job is to do what I enjoy doing. If anyone else thinks it’s valuable, that’s cool, but I am not depriving them. Quite the contrary. I’m being polite and respectful.

I guess I’m saying that selling shouldn’t have anything to do with it. We’re people. We’re living our lives. We have no obligation to sell ourselves to anyone. If people are interested in what we do, fine. But it is usually extremely annoying when someone pushes themselves on you uninvited.

I have in the past, sold myself short. It is as @janbb says. Not valuing oneself enough. Basically, letting people walk all over you.
I don’t let them do that anymore though.

To fail to go back on the stand and deliver your last useful information to a jury during a civil trial, for example.
In my case, A BIG SELL SHORT.

It means that you chop off your legs and then put yourself on Ebay. :) Buy it now. LOL

I’m thinking it means don’t under-estimate yourself. Or look down upon yourself, IM just guessing.

I hear this expression most times when it came into relationships that involved sex. One of the persons involved had no clue whatsoever what was there to be done into improving the relation. But because they always enjoyed the time spent in bed the other person sold short so they can enjoy that particular time.
[ but this is just what I saw and heard ]

It could also mean betting on yourself to be undervalued and making your choices accordingly.