Looking at the eBay page, it would seem that you have a 30 day cash back policy so that’s good news and as for the shipping from comparing prices it looks like it will be about $45-$55. Get your money back and build your own pc cheaper and you get to build it around you.
Yeah just message the seller and tell him of your intention to return the item and if he gives you any guff, call ebay and paypal and open a case and follow their return instructions. You have seven days from the day you recieved the item to return it and no matter what the seller says, you are protected. You will get everything back except your shipping. That 7 days is extended in your case according to the sellers return policy which ebay requires them to honor!
The seller I linked above even refunds your shipping for upgrading the gpu and I have also known him to refund shipping when Item returned.
He's just a nice guy like that!!!