I think I could probably give you better advice if I could see the original listing.
But, you do have two angles you can argue if you or the Buyer ask eBay to step in and decide the case.
1. Did you deliver EXACTLY what was described in the listing, in a timely manner, to the Buyer? If so, you have fulfilled your obligation to the Buyer.
Discussion: when presenting this argument to eBay, you must do your homework because the eBay reps will not know anything about the technicalities of processor chips, you will have to hand-hold them every step of the way. You must explain and document that the particular chip you sold does not support on-board video, and the fact that (a) the Buyer did not know that, and (b) Buyer's principal complaint that when installed nothing comes up on the screen at bootup, actually PROVES (a) the Buyer did not do the proper research and assumed that your chip would work in their motherboard when it in fact will not, and (b) the as-described processor chip is incapable of generating video. Again I have not seen your listing but it would be a BIG plus if your original listing stated that the chip does not have on-board video and that a separate video card is needed with that chip. You have to get across to the eBay decider that the problem here is not that you sent a bad processor chip or decribed one type of chip and sent another----you have to get them to understand that the problem was that the Buyer did not know enough to select the correct processor. This argument can be made stronger if you can find out what model of motherboard the Buyer is using in their computer, and demonstrate to eBay that particular motherboard and processor chip combination requires a separate video card.
Then try to come up with an analogy they can comprehend---say the item was a clothes iron. This is like a person buying a clothes iron and assuming it has a steam feature when it does not have it. Because the Buyer can't get the iron to produce steam is not because the iron is broken, it's becasue they bought the wrong iron for their needs.
2. The Buyer removed the item (processor chip) from the original factory sealed package and installed the processor into a motherboard, then into a computer, and then fired it up. Therefore, the item changed from "new" to "used" and thus lost value. Not to mention that the item returned by the Buyer is significantly different from what was sent.
Discusssion: This argument can be bolstered if you can show that your particular processor chip sells for a significantly lower price when it is used versus when it is brand new in sealed factory packaging. Another approach is that when you are presenting your case to the eBay decider, turn the tables and ask the rhetorical question, "How would eBay have handled this if I had listed a brand new processor chip and sent them a used one in an open box? EBay would not like that and find for the Buyer, right? So how is it OK for the Buyer to send me back a used processor chip?" In eBay-land, this is a weaker argument because eBay will likely counterargue that you as a Seller have to be prepared to take losses like what happens when a new item comes back used. You can try to counter-counter by stating that it was clearly the Buyer's error (see #1 above) that created the issue and all you did was provide the EXACT item that you described in the listing.
IMHO, forget about taking anyone to court or even mentioning that when speaking or writing to eBay. They must hear that a hundred times a day. Also, refrain from speculating about the Buyer's motives, concentrate on what is obvious---that the Buyer incorrectly chose your chip for their application. Concentrate on emphasizing that you delivered the as-described item in a timely manner and the Buyer's error is what created this problem.
Good luck, you're gonna need it!
