Real Dictionary

sell out
phrasal verb: If a performance, sports event, or other entertainment sells out, all the tickets for it are sold.
phrasal verb: If a shop sells out of something, it sells all its stocks of it, so that there is no longer any left for people to buy.
phrasal verb: If you accuse someone of selling out, you disapprove of the fact that they do something which used to be against their principles, or give in to an opposing group.
phrasal verb: Sell out means the same as sell up.
phrasal verb: When things sell out, all of them that are available are sold.

sell out in American English
to dispose of entirely by selling
to get rid of completely by selling

sell out in British English
to dispose of (supplies of something) completely by selling
noun: a performance for which all tickets are sold

sell out in Retail
verb: If a store sells out of a product, it sells its entire stock leaving none for customers to buy.

sell-out
If a play, sports event, or other entertainment is a sell-out, all the tickets for it are sold.