First, it is not a firearm. You did not fill out a 4473 when you purchased it. Second, it is not a machinegun. Once ATF communicated with you and "confiscated" whatever part they felt was the offending part, it pretty much went from "machinegun" to a pile of useless parts. Since ATF declared it to be a "machinegun", you can be sure that they would not have let you keep any parts that were contraband.
If you want to keep it, keep it. If you want to sell it, sell it. Problem is, that it is essentially useless without the required parts. It's only worth something to someone that either collects stuff like that, someone that will convert it back to full auto and do a post sample or someone that wants to try and re-engineer it and have another stab at ATF with it.
I would be interested in it, with the goal of re-working the getup, and trying to have it approved by ATF. Problem now is, that it's not worth much, it is after all, "missing parts" and not usable.
Value? Id think something like, an AC556 parts kit, minus the receiver, minus the barrel, minus the bolt and minus the select fire components.