You will have to figure out the primary wire pairs by trial and error with a DMM, as described above.
No Variac, no problem.
Put a 40 or 60 watt INCANDESCENT lamp in series with the suspect primary. If the lamp lights-up dimly or not at all, congratulations you have found the primary.
Now with the DMM figure out all of the secondary voltages and its relationships.
I.e. a secondary may be center tapped.
As others have mentioned, if you show that information in your sale listing, chances will increase exponentially that someone will become interested and make a reasonable offer.