Update 1
Update 2
Back seam is easy to fix in Gimp. Just offset the head horizontally by half the size of the canvas, and make sure wrap is checked. Then fix your seams and offset again.
It's worth 800tc. You might be able to sell it for 2k if someone falls in love with it, but that's unlikely.
To make the visor more visory try highlighting it like so (Don't copy this method exactly - take bits and parts and deviate in your own ways):
You have some interesting shapes there, but the texture itself lacks contrast. The bulk of the head is a single grey, which honestly is boring. Try mixing in some color, or try using different shades of grey to make some plates stand out over others. Doing either of these should make the texture more eye catching, hence making it sell better
Also, I'd remove/change the plates that are tangent to the visor. Tangent lines kill perceived depth. Right now those plates and your visor are on the same level, making an awkward visor strap around the head.