Confuse... Your head is too beginner and but the colors and mapping worked good.
Selling it? You can sell it to another beginners, not surely about the prize. but 1,5k is the max.
This is worthy of selling of course.
No more over 2.5k imo
Maybe more if you fix a few things.
Like the odd shape seam in the back and add a few things.
The back needs work like you said, but if you raise that little flap in the front up a few inches and add some eyes, and maybe a robot mouth with tubing coming out of it circling around to the back, that fixes the back problem, it would be worth of a fair amount of money. I really like it!