I make a spreadsheet of everything I send and match it up to the one they send me when the cards are added. It's time consuming but it works. With the cards already on the site, you can make a spreadsheet using the bulk editor which is available in one of their old blogs and it still works (not sure when the old site is closed if it will work)
You can also use the "List" sort on your own cards and just screen print or copy that to excel I suppose too.
There is nothing stopping someone from taking cards in your port and selling them elsewhere all while being listed on COMC (IE same way comc also lists on amazon). What happened to the other member was not that, comc never actually had those cards in hand.
No one should ever have one of your cards in hand and have it listed elsewhere as they should all be in their warehouse/building.
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RIP Mom 10/8/09, PAPAJIM 11/22/10