I am a newbie in SEO but earlier this year, I experimented and bought 5 PBNs from a reputable seller for a money site I bought as well. Turned out the SEO experiment has not been successful so far, money site is not making money. I am leaving the money site on the side for the moment as I am focused on a more profitable venture. I am not too sure of what to do with the PBNs and they are costing me money (hosting + renewal of domain names). I am thinking of trying to sell them. What do you guys think I should do?
You could sell links from them on SEO/Internet marketing forums, check out Blackhatworld/Digitalpoint/Warriorforum/other SEO forum's marketplace, see what links from domains with similar DA/RD go for, and launch a thread. Give away 1-2 review copies and start printing money 
Thanks! That's very helpful. Is there a limit on the number of links I can sell per PBN ?

To be able to sell links on your PBN, people would want to see some ranking proofs. If you can pull that off, you are good to go. A permanent homepage article link (not site-wide blogroll link) usually would cost much more than a link that rolls into the inner page. Good luck!
PM me some of your domains, I will get back to you with my two cents on their quality.
Tbh, I haven't heard much about someone "buying PBNs". Usually this means that dozens of thousands of USD are being moved around. In my world, we usually talk about domains for PBN, which can go between $70 to $200 per. If the domain is live, has unique and relevant content, as well as some domain authority - Ahrefs, Majestic, Moz - then it can go for a few thousand. Again, depending on the niche.
If it’s difficult for you to support them, as far as I know, this requires constant updating of content, updating backlink profile, etc. - better sell them.
Google knows more about us than our mother, therefore, with tools like PBN you need to be very careful.