Hi David,
From your explanation, it seems like a straight-forward enough scenario.....
...You order goods, you pay for goods - nowt arrives.....
...You contact seller, he agrees you have paid (or at least, he nevr queries it), and item was returned to him...
...Job done - he agrees he has your money and your goods - evidence complete (hopefully you have got access to the messages between you).
It is the next bit which gives options...
in the 'real world' we all accept such things happen, and the offer to re-send etc etc is acceptable. When nowt happens we ask for a refund - again nowt happens.
Okay, so we have a lot on our (your?) side here. Firstly, the 'seller' accepts that you have paid, and not received your goods (Bank transfers are obviously recorded), he offers a refund (which is an admission of failure of sorts), and it never arrives (again, EBT records are in your favour).
The options I would consider (in no particular order) are:-
1) Ask for a refund by a certain date.
2) Small-claims court
3) Old Bill
4) write it off
I presume you are now fed up with trying, hence you make this post, so (1) is out of the way....
(2) - lots of hassle - and you need a few things to go your way (but may become worthwhile in a bit).
(4) - bugger-it, why should you!
so I'd go with (3).....
I can assure you that the local Police will probably be less than thrilled at the thought of this report landing on their desk, but every so often you find a copper who thrives on such things and takes it as a personal challenge to win - either way, despite what they may (at first) say, it IS a Police matter.
As an insider secret (which isn't really a secret, and I'm no longer 'inside'

), Police haven't got enough time/people/money to investigate everything, so they try and palm-off some things, and tell you it's a civil dispute.
In the olden days (pre-2006!) this case may well have fallen into such a category as 'Deception' needed to have an admission of 'thought/intention' to prove...
...but since the Fraud Act came out, it is now enough to show that an act or ommission by one person has cost another one something - Simples!
As he hasn't disputed the facts, he is (at this stage) extremely suspicious of commiting Section 2 Fraud - as such, the police have a duty to investigate it.
(as Ailwyn said - get a crime reference number at the police station - even if they say they can't do anything, ask for it to be recorded - they have to!)
The next stage is that there will be an anguement as to who should investigate it - I assume it isn't a local seller or it would have been collected???? - so the matter will go to the Police force where he lives (probably). Maybe the local guys will give him a call to try and encourage him to pay up quickly, but they'll probably just pass it along.
It will take a few days to transfer, then someone from whatever police force gets it will be in touch, and give you their reference number (which may well be different).
It might just be a case of the copper 'having a word' with the seller, which gets your money back with no further action.
If it doesn't happen like that, then it CAN lead to his arrest/interview for Fraud, and maybe a charge to the Magistrates Court. If it goes that way, then the Court will make a compensation order in your favour. These cannot be easily enforced, but make a great base for a civil claim.
This is a bit of a 'worst-case' scenario, and rarely gets that far (but the theory is the same for any Fraud offence) - I would expect a quick call from the Police to resolve the matter fairly quickly with the cash back into your bank, or at least, a specifed timescale for it to happen.