Is there a cross reference for GM auto part interchange numbers?

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Most all of the sellers I've seen offering used GM auto parts for sale lists an 'interchange' part number.  Since I know what the OEM GM auto part number is that's associated with the part I'm looking to buy,  I can't seem to find any way to verify these interchange part numbers with genuine GM parts that these sellers are listing on eBay.   Most every seller includes only  'interchange' part numbers and not OEM part numbers. These sellers were able to find these numbers somehow, yet I don't know where they came up with them.  It's easier to find an OEM number on the internet than it is to find an 'interchange' part number.

I also don't know how to verify what the fitment is for these interchange numbers. Sellers are listing different part numbers for the same component and fitment of vehicle, (same make, model, and year).  One seller offering the same part posted a picture of the GM number embossed right on the part, except that part number is incorrect for the vehicle fitment. It makes a buyer beware of every seller offering for sale genuine GM parts. 

Does any one know how to cross reference an interchange part number to verify the genuine GM part numbers for a particular vehicle?  (besides asking the seller to provide the number)