What is good is that now you have the identity of the seller, so what you need to do is to take the matter to the next step. Indeed, whether the seller is a cheater or not, as a buyer of second hands product you have some rights and there are ways to get them enforced.
If that would be a buy from a business, then you would have 14 days after delivery to cancel the contract (info in spanish here: http://cecu.es/especiales/14dias.pdf
As I understand, this is a buy from a private person, so here spanish law says your rights are unfortunately not protected by consumer law (so, no right to cancel within 14 days), but at least your rights are protected by the civil code.
That means that you can still cancel the contract if the good does not correspond to what was announced, or you can get a compensation for hidden defects. In order to get this, you need to go to a tribunal. You have 6 months to proceed, counting as of delivery day (see here, in spanish: http://www.cyzabogados.com/compra-venta ... ticulares/
I understand that going to a tribunal in particular if you are a foreigner (possibly not speaking spanish) is not the most appealing road to consider. However, there are intermediate actions that you can take before that, which might solve you problem. One easy thing to start with would be to have a lawyer sending an official letter to the seller asking for the devolution of the money (against the devolution of the guitar), and stating that in case of refusal a court proceedings will start.
Knowing that you have good chances to win the court case, which is gonna cost to the seller, the seller might think about it and offer some form of settlement. A possible settlement would be that he sends you another guitar with some real value, that he already has in his possession, which could help you making up your loss.
There are plenty of online lawyers companies in spain, i have used one and this company was working well and cheap. This will however likely cost you some money (say something between 100 and 400 euros, assuming a rate of 50 euro an hour). Another option would be simply to write the letter yourself in spanish and send it to the seller, with a deadline to comply to. In the absence of reaction, then you would need to contact a lawyer firm and they would take care of the procedure (which is likely not gonna cost you as much as you fear, and probably much less than what you paid for the guitar).
That is for the legal road. Then you might also try other non legal options. I understand that the seller seems to be interested in guitars, and is actually quite knowledgable. So, very likely he is connected to many guitar buyers, sellers and players, and you can try to identify members of his network one by one, explain your story and ask if there are ways they could help you. He might get to hear about your attempts, and if you end up contacting someone he cares about (for instance because he has a business relation with the person), he might not like the negative publicity and offer you some settlement. You could of course also target other social networks, like the spanish version of delcamp.
These are just a few hints, just to say that you are not completely powerless. I am happy to help with the spanish if this is one obstacle for you. Hope this is helpful.