Its a double imposter scam. Happens quite a bit on Sythe and people are caught out because of the nature of this scam.
1. The first thing is that you traded an imposter and not Service RS which means he is not liable.
2. Secondly, even if he was to make an imposter account you would have to prove it beyond doubt.
When looking at the PM in the future ensure this:
1. The PM contains a part of the Skype conversation (IT SHOULD BE EXACTLY THE SAME) - Sometimes you can notice the messages are very similar but not the same
2. Majority of major RS service providers have a Skype button somewhere which you can click. The benefit is that it will add the correct person automatically so no risk is involved.