Developed initial version of website in belief contract would get signed. They backed out. Is the site mine?

The title is probably my best summary. It is similar to this other unfortunate scenario, but I ask here because of a few differences. So here are the mains:

• Two months ago I sent an email to a guy on upwork who was looking to hire out a programming job I was uniquely qualified for. In my email I linked him to my proof of qualifications.
• I received a phone call from him where the first words he said were "Where the fuck have you bean!? I’ve been looking for you for months." He went on to tell me how he was loosing faith in his project because he’d already interviewed over 30 developers who didn’t fit the bill. Then said he got "goose bumps" when reading through my blog posts because I showed that I’d basically already done in a prior project what he was trying to do (albeit for a different application in a different industry). At some point I said "If I were to be the one to do this job for yo.." when he abruptly cut me off to say "No no. There is no if. This job is yours." I was feeling pretty good.
• Over the next two months him and I discussed the project with his partner – the sole financier of the project. Communication wise:
  • They shared with me: very general block diagrams of how the system would work, examples of websites they wanted to emulate (which were gross), and a CSV of people they wanted to advertise to obtained from public government records. They never provided code of any form to me.
  • I shared: outlines of a better system, better 3rd party providers to integrate with, and educated them on machine learning.
  • I learned that the financier, who is involved in a hedge fund, is litigious – he would gloat about various ways he’d made money including a story about making $50k by buying pre-foreclosed real estate who’s mortgage he knew was improperly recorded so he could sue the city/bank to nullify it.
  • These communications spanned about 30 hours.
• We negotiated through several revisions of a written contract that ultimately was not signed. The last revision I requested was that I would retain rights to all works until receipt of final payment, at which point I would transfer over all rights.
• Both partners told me multiple times that they wanted to hurry up and get this contract signed so that I could get paid/start.
• Communication from them slowed last month… I was told that the financier fell ill.
• Two weeks ago I told them that I would immediately begin working on the project "full time" in anticipation of finalizing the contract. It was clear they received my message because the financier responded saying he would call me the following day. He didn’t call me, but I did develop a working version of the software. It is live on a domain name I own. All the visuals, design elements, and code were created by me. I figured branding could easily be swapped.
• I updated the financier that I’d created a first version and was going to have beta testers using it this week. Then he sent me this:
  

So my questions are:

• If this guy sues me, what could he win?
• The initial idea for this site was theirs but I am the one to manifest that idea into a usable thing using hard won know-how and assets. Is it completely mine to monetize?

His abrupt change of heart has left me with about 80 hours of non-compensated work as well as server and 3rd party service costs. So about $7k in currently wasted effort. If it is okay for me to sell this, should I first offer it to him? Or should I not speak to them anymore?