Option to leave product stock counter on purchase

-- url of the page with the problem -- : www.schmuckdesign24.de/
-- HikaShop version -- : 2.3.5 Bussines
-- Joomla version -- : 3.3.6
-- PHP version -- : 5.3.29

I manage a shop where all products get manufactured on demand /purchase.
When somone purchases a product I want to leave the on stock counter untouched.
I have tried some options but didn't find the right option.
Is there a possibility to change the shops behavior as described?

Just now I always have to set the stock counter back to 1 if someone purchses a product.

Bruno wrote: I manage a shop where all products get manufactured on demand /purchase.

You got it right!
So where can I set the quantity to unlimited? Checkbox or anything like this.

... I tried to enter unlimited (Unbegrenzt) into the quantity field ... and it keeps the value.
If this is the correct way, it is not very sugestiv.

I just found out: empty field or anything else than a number is the same as unlimited (Unbegrenzt)

Bruno wrote: I just found out: empty field or anything else than a number is the same as unlimited (Unbegrenzt)

After inspecting the hikashop database tables with phpAdmin I found out that unlimited (Unbegrenzt) means that the value of product_quantity in the table jos_hikashop_product is "-1"

Now I changed all products of the shop with this SQL statement:

UPDATE `jos_hikashop_product` SET `product_quantity`=-1;

Thanks for the hint, I will try the mass-action system next time