The definition of an exploit:
A software tool designed to take advantage of a flaw in a computer system, typically for malicious purposes such as installing malware.
The definition of a zero-day vulnerability:
A zero-day (also known as zero-hour or 0-day) vulnerability is an undisclosed computer-software vulnerability that hackers can exploit to adversely affect computer programs, data, additional computers or a network.
Is it illegal to create and sell a exploit of a zero-day vulnerability?