As already said even if seller has agreed a partial refund, I think that speaking to ebay would be a waste of time, as they will not get involved.
Have you left feedback, or alternatively if you have a problem with a purchase open a NOT AS DESCRIBED CASE, for a return and full refund. This might encourage seller to respond, but promises are not enforceable, and if you close the case you cannot reopen it.
Remember that once over the 30days, no case can be opened, which is probably what seller is hoping will happen.