"Selling" is usually associated with overstating of the qualities of the thing you want to exchange, in order to increase its perceived value. This is considered, for some reason, a good thing, an achievement or desirable skill or a social norm. When everyone strives to master this desirable quality, each exchange of a thing artificially bloats its value, while the things probably remained unchanged since the first exchange. That doesn't make sense.
You don't need to "sell" anything. Having to sell yourself, whatever that means, is pathetic. If you have something of value that you want to exchange with some other thing of value from someone, just communicate the facts/specs about the thing you want to exchange.