How to utilize a targeted email list?

Hey there,
I am currently a freelance developer getting jobs from many local companies and they trust me that basically gives everything to me to handle. One of my clients is a big organization with many events for others to join about mental health and mindfulness and so on. So because of that I actually get a very huge contact list (thousands of it...) with ppl's name, email, phone no and even home address.

So one day I was just thinking, any chance to use it for some extra cash? I dun dare to sell this type of data directly to someone else coz I'll get into deep trouble if being discovered.. any safer way to use such thing?