I appreciate you explaining, and maybe there is something I do not understand here, but surely you are making very good profit from each product batch you buy? Therefore that would make testing a very insignificant % of profits?
Therefore, yes, testing everything will cost much more than not testing. But the cost gets easily absorbed by profits. No?
Surely you realise the value of testing. If so, you must also understand that if any vendor starts testing every or most batches before selling them, then customer will flock to them. Leaving all the other vendors behind.