Ok. I understand what I was doing wrong. My form element "hbCart_1_stockAmount_Add" was set to "hidden" with the default value equal to rsBooks.stockAmount - as you stated.
However it is still not doing what I need.
If my inventory (stockAmount) is 1, it will allow me to add 1. But if I try another higher number, validation fails.
BUT here is the problem- I can go back and click the add 1 to cart repeatedly in increments of 1, so my cart can then have more than 1 of the item (over my inventory stockAmount). Passes validation
The validation is not taking into account how many of that item are already in the cart. Is there a way to modify the validation to accomplish this?