Mine is an excellent issue two, not boxed but c/w leads, p/supply manual and extra books set me back £100 postage inclusive.
The trouble with small volume projects like this is that they are never cost effective when selling to others and then on top of everything else you then have to find a suitable case,transfers etc if you want it to look original.
Also the ZX80 is so niche that the few people that actually use them will probably have the real thing anyway or be satisfied with emulation and collectors only want the original computer.