Sold Out and Selling Out: Very closely related but completely opposed to each other.
According to The Free Dictionary,
SOLD OUT is defined as 'having all tickets or accommodations completely sold, especially ahead of time'.
In how we live our lives personally, in practice, and in business, SOLD OUT also means that we have surrendered completely to a set of guiding values and principles.
SELLING OUT on the other hand, according to Wikipedia, is defined as 'an expression for the
compromising of a person's integrity, morality, authenticity, or principles, in exchange for personal gain, such as money.'
In terms of practice or business, selling out may be associated with a drifting mission or a vision to make oneself more mainstream or appealing to a wider audience.