[judge2020]

It is illegal if the website’s TOS for making a purchase prohibits the use of automated software.

It doesn't matter if it's legal, it matters if the website owner doesn't want x doing y on their site. A bot consistently not abiding by owners' intent is inheritly malicious.

[ikeboy]

What law does using a bot to make a purchase against ToS violate?

The LinkedIn case shows that it's not illegal in some cases - would you classify the behavior of the defendant in that case as malicious?