Look up the owner using Whois. There has to be a valid email address that goes to the owner. Even if it’s under privacy it must still work.
Emails that don’t work is invalid whois and can be reported to icann. Registrar has to investigate for invalid whois data or the domain will be made available again.
So to make an offer just send an email. If they’re interested they’ll most likely get back to you.
Most registrars will also get in touch with their customer to see if they want to sell.
