Ebay Reselling

I acquire items and sell them on Ebay. Some items I purchase individually, while others are purchased as a group of items. I do not keep track of inventory and I use the cash method of accounting. When I sell the first item from a group, I use the entire purchase price to calculate the COGS. In some instances, items from the group may be sold over a period of several years. For any items sold afterwards, I no longer use the purchase price in my COGS calculation. Is this an acceptable way of calculating my COGS?