The ROM is likely a regular ROM, with just the Mew shared via trade. Besides ROM sharing isn't allowed here anyway so that's off the table.
(and, even if the ROM were to be different, I don't expect it to be different from the debug variants that leaked a while ago)
As much as having an old Mew would be interesting, it's not difficult to fake one. Without adequate proof (such as old certs and photos and a convincing story), it would be ill-advised for anyone to make such a purchase (because more often than not, these Mew are fake; they are very easy to fake).
Also, quite curious this seller mentions save, rather than cartridge.
In any case, if anyone would like to do it, just know to do it at your own risk. I'm just being a cautious bystander and warning everyone of the associated risks, carry on.