I'm a professional seller on eBay and am coming to this thread a bit late but thought I'd add my 2c.
Firstly, I agree with most here and would have backed them in a heartbeat. If the seller's profile/description *didn't* say 'Returns Not Accepted' I'd have simply sent them straight back (same day if poss.) and then after they were gone, told the seller they were being returned for a refund.
If it did say 'Returns Not Accepted' then you have to go through the dispute system as OP has done.
The problem with the dispute system is that you've only got 7 days to get all your ducks in a row and make sure eBay have *ALL* the info that you think they might ever need to find in your favour. After the case closes, you can't then go back and add anything like email conversations or further commentary.
The other thing to bear in mind is to be very careful what you log the dispute reason as. This is because if eBay manages to find a way out of it, they will (much like insurance companies). You have to give them all the info and also, no wriggle room to find a way out of coming down in your favour - it is quite difficult.
OP: You seem to have come out ok this time due to your persistence - I gotta admit I'd have probably given up before getting the $300... Saying that though, I'm more likely to have travelled to sellers place and gotten hold of him as I'm not a patient man ;)
Edited by garysan - 9/14/12 at 2:24am