The unusual part of your situation is that you were able to use the Mac with your own Apple Account for approximately three years, and it appeared among your devices, but after you erased it while preparing it for resale, Activation Lock displayed the previous owner's email address.
The first thing to establish is whether the Mac is currently associated with your Apple Account in Find My.
Check Find Devices on iCloud.com.
Apple explains that erasing a Mac does not by itself remove Activation Lock; the Mac must be removed from Find My. Apple's instructions for Activation Lock on Mac
If the Mac is still associated with your account, follow Apple's instructions for removing it from Find My.
If it is no longer associated with your account and the Activation Lock screen displays the previous owner's email address, that would be consistent with the Mac being associated with that Apple Account for Activation Lock.
If you can contact the previous owner, you can ask them to check whether the Mac remains associated with their Apple Account or Find My.
If you cannot contact them, Apple provides an Activation Lock support request and states that proof of purchase is required.
Apple Support — Activation Lock removal
Since Apple has already declined your requests because you have a third-party receipt rather than the original receipt, I would ask Apple specifically what documentation they require in your circumstances before submitting another request.
When contacting Apple again, make the unusual history clear: you purchased the Mac approximately three years ago, used it successfully with your own Apple Account, and had it listed among your devices. The previous owner's email appeared only when you erased the Mac in preparation for resale.
Apple can then determine what that history means for the current Activation Lock and whether your available documentation is sufficient.
