I've seen a few sites like this now, but I don't understand how you can buy things from ebay like this.
We accept Bitcoins, and purchase the Ebay item for you automatically using the Ebay API. The only other site that I am aware of that functioned in exactly this manner (although manually, rather than through API) was Bitspend, however they appear to no longer be operating.
How do you handle ebay disputes, returns and refunds?
We handle all disputes in our marketplace, ebay or otherwise, in the same manner. Each ebay item has a specified shipping time; after this time, there is a 7 day grace period, during which the purchaser may file a dispute (and, during which we hold the funds in escrow if the seller is not from ebay). When a user files a dispute, we will relay that dispute to ebay if we need to. After the 7 day grace period has expired, if no dispute has been filed, the funds are then paid to the seller irrevocably and the customer loses the ability to file a dispute.
In the case of an Ebay refund, we will refund our user upon receiving our refund from ebay.