i think that the problem with domain names flipping is it must be done properly. i dont know if there is some specific methods to do it. but you must not rely on you own personal taste, you may like a domain name and tell yourself " wow, how a domain name this good is still available till today ", but in reality no one is interested in it. just my two cents.
if you want to go ahead in do domain flipping business, go to the domain auction websites like sedo and make a tour, you will have a better understanding of what makes a domain name valuable and what people really want.
For your existing domains, it's up to you to renew them or not. maybe some of them are hidden gems, maybe not.