I would never buy memory on Ebay. Buying anything on there is a chance. Memory is something best left to a reliable local store.
Well you are certainly missing out. I bought the memory from a dealer and there will be no problem returning it. I have bought computers before on eBay and they all contained memory. No problems so far, apart from this one and I have been a member of eBay for over seven years.
In my experience the simplest way to buy memory is use one of the manufacturers websites. I usually use Crucial which has a detection tool which detects what I have and makes the sale simple and fast. The trouble with DDR2 ECC FB memory is that it is expensive, over 230 GBP for eight GBs so provided the memory works I am quite happy to buy second hand.
While purchasing memory from ebay can save you money the statement "let the buyer beware" certainly applies.....it is up the the buyer to do ALL the research related to the item being purchased.
What you bought is server memory, not desktop memory and I agree that the specs posted meet your needs, but, I doubt that this posted specs are correct (see the memory I did find on tiger direct at the bottom of my post).
I also have purchased memory/drives/computers from ebay and have had mostly good experiences provided I stay with highly rated resellers but I also did the research on what I was buying and did not take the resellers word for compatibility. Computers that are sold on ebay have compatible parts because they are a complete units and not bare bones kits.
As to parts the reseller may be mistaken about the item's compatibility or taking someone else's word for it, or as pointed out, may be deliberately mis-representing it to get rid of a unsellable turkey. Remember that most resellers are not computer/hardware tech's...they are simply saleman selling a product, no different from a used car salesman except you can't physically handle the product so it is bought "sight unseen".
As I stated before a little research will show you that dual channel (DIMM) DDR-2 is limited to 2 gb sticks and that you would have to go up to DDR-3 to find 4gb sticks for a desktop (ddr2 @ 4gb is available for a laptop).
I have found just one(1) item that approaches (but does not meet) your specs @ $214.99 per stick but it is not dual channel memory (DIMM per your specs), does NOT have full length heat spreaders, and is NOT fully buffered:
Crucial 4GB PC5300 DDR2 667MHz Desktop Memory Upgrade at TigerDirect.com