Store's unseen items sales forecasting
If you use R the package forecast provides neural networks algorithm as nnetar() function and it has xreg option. You could use your stock data as regressor. Examples and details: http://pkg.robjhyndman.com/forecast/reference/nnetar.html

Or you could use var models because there is relation between stock and sales e.g. if you have low stock your sales would be min(y,stock) where y is the true demand to your products. It also applies in other direction too if you have high y then you will be out of stock fast. 

But I'll suggest you to take look forecast package function nnetar() (in R).