Determining cog on items for taxes

[mommaknows]

I own/owned many things that I have listed and/or sold recently and being a very new Ebay seller want to understand how you all figure COGs for taxes on these type of items. Is there some sort of formula? I am trying to get all my ducks in a row early on. 

Any and all advice would be very much appreciated. 

[frodobagginskennedy]

When we met with a CPA several years ago he said to take 25% of the sold price to determine COGS if we do not have receipts.  Hubby is VERY conservative.   We use 20% if we do not know.