Error Metric for Prediction of sales quantity (by using Attributes of item)
This is a fairly common task, especially in fields like economics, but the choice of using a Neural Network makes your approach different from most.

Having said that, I think you can still use common validation metrics:


Mean Error
Mean Absolute Error
Absolute Error ("delta")


You probably don't need anything more fancy than that, especially if those are the error metrics that your business clients will be using to measure your success (which I imagine would be the case).

Having said that, if you want a fancier ML metric to use whilst developing your algorithm you might try Root Mean Squared Error (RMSE). Here's an article on the topic.

If you wanted to, you could also construct a pseudo-R-squared metric, but those are less used with ANN's and I wouldn't choose it myself for this particular case.