Yes, "take it easy" is an informal way of saying "good-bye". You would only say it to a friend. There are many ways of informally saying good-bye. Here are a few: bye, bye now, ciao, bye-bye, see you, take it easy, take care, take care of yourself, have a good one, see you soon, have fun, go for it, you go girl, have a nice evening, good night, adios and so on. The good news is that "good-bye" and for friends, "bye" or "see you" are all you need to be able to say.

There are all kinds of expressions like this: Mind how you go. Take care. Be lucky. You would usually say a more normal goodbye phrase, such as 'see you', before this phrase. The intonation and body language would also make it clear that it's a farewell.

'Take it easy' has different meanings depending on the tone. It can be used as a casual farewell if said in a particular 'farewell' kind of tone. In this way, it's not meant to imply anything more than a well-wishing farewell. Similarly, people can say, 'Don't work too hard,' but this is sort of a more joking kind of expression (especially for people like security guards wherein they are often just sitting or walking around). Blue-collar workers may often talk like this, but it can also be a sort of California like thing too. If it's said more declaratively, then 'take it easy' is more like an instruction to the person to calm down. If said in this way, it can almost be mildy insulting or admonishing. 'Take care' can also vary based on tone. If said in a casual, farewell kind of tone, it just means 'goodbye,' more or less. If said with a harsher tone, it can sound almost threatening.

‘Take it easy’ is not the equivalent of ‘Goodbye’. It’s a parting message that you would use to convey that you think the person you just talked with is working too hard or under stress, for example. You wouldn’t randomly substitute it for ‘Goodbye’ which is neutral and doesn’t have any other meaning.