Something doesn't quite sound right about this........a receiver has to have a serial number, or Uncle Sammy won't let it be until it does. If you're buying something un-numbered, then it's not finished, and if you do they final finishing and make it into a working gun, now you're a firearms manufacturer, subject to taxation and a gazillion laws that BATFE will be happy to shove up your keister if you dare transgress.
Sounds dangerous............or a trap. Make haste slowly. :P