Sell Out Meaning

(idiomatic) To abandon or betray one's supporters or principles to seek profit or other personal advantage.

Example: To sell all of a product that is in stock.
  They sold out of concert posters yesterday and won't get another shipment until next week.
  It used to be a pleasant little community, until the leaders sold out to the developers.
To betray a person, usually a close friend or family member, for personal gain.
  I'll kill him if I see him again, he sold me out.