Retailer Price Setting for Existing Inventory in the Event of an Increase in Price Paid for the Good If you are a retailer, and the price you pay for one of your retail goods goes up, how do you price the inventory you already had (which you paid a lower price for)?

Here is an example of a TFD (True, False, Debatable) type question for this idea:

Suppose you run a hardware store, and the price you pay for sidewalk de-icer increases. You raise the price on the remaining stock of sidewalk de-icer in your store, for which you paid a lower price.

I think the answer to this question is false. Increasing your price on the inventory you already had violates the PMP, so any other price would lower profits.
At the time period in which you buy the new retail goods, the cost of having bought the old inventory goods is now a sunk cost. Your pricing decision I don't think should be based on the price of the old inventory. The new point where marginal cost equals marginal benefit (assuming competitiveness) is now the price of the new stock (more expensive).

Without competition though, the result will probably change, since the quantity you supply will now matter in affecting demand.