WordNet records no synonym and no antonym for sellout. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
an event for which all tickets are sold
“get your tickets for the concert early, it is sure to be a sellout”
the selling of an entire stock of something
Synonyms, antonyms and the broader and narrower relations for sellout come from Princeton University’s WordNet. Full sources and licences.
“the latest Xbox game is so popular, shops are warning of a probable sellout before Christmas”
a betrayal of one's principles principles, country, cause, etc.