Why is it inappopriate to post goods puchased at the time of purchase instead of the time actually sold??

That's not true. That's actually the correct way of doing it. When you purchase your inventory you should Dr. inventory & Cr. Your payable and Dr. COGS and Cr. Inventory when you sold the goods

The question is seven months old.

But for anyone reading this...

The person titled it "posting to cost of goods sold." Meaning they were asking about why it's incorrect to post to cost of goods sold when it's purchased, not post to the inventory account. And posting to cost of goods sold at the time of purchase is indeed incorrect.