The Flux Capacitor is an interesting device. While I'm no expert, I do believe there might be more to it than it seems.
But... We know what's inside the device, as we have the
schematics available on the forum and elsewhere on the Internet. The FluxCap contains about 10$ of electronic components. Plus the box, that would be about 25$ total, maybe? As TimeFlipper said, it costs almost nothing if you don't buy a project box.
I have most of the components, as I tried to build it more than 10 years ago. It was fun and interesting to gather the parts at my local Radio Shack and build it. Naturally, I got some of the parts wrong, but that's another story!
In other words, don't pay 380$ on Ebay for a device you can build yourself for less than 25$, without much electronics knowledge.