iMac activation lock 2019 model

I got this thing from some guy off craigslist. He showed me it working in a Starbucks. So I gave him the $800 he was asking for and took it home. Logged in with the password her gave me and attempted to press control r to reset everything. Boom I get presented with a padlock. I then restart it and log in again. Plug it into a Ethernet port and then it resets itself . Now all I am presented with is the ego lock I believe is what it’s called. How can I fix this? Apple will not do anything for me since I don’t have original receipt and now the guy is not replying to me to even attempt to get the receipt from him. Am I SOL? Can I just change out the logic board or is there an easier option?