Why stop there, you could buy real shares, sell synthetic ones, and then sell the real shares, too. Or just sell synthetic shares without having any real shares in the first place. This seems like a case where a blockhain doesn't really do much good.
Selling the real shares as well as the synthetic is really just going naked short. Lots of risk/leverage. You have to put up collateral so that you have something to lose if it goes bad
I'm looking at it from the buyer's perspective. The big selling point of blockhain transactions is zero trust and no counterparty risk. There is clear counterparty risk in synthetic shares sold on a blockhain.
IOW, the buyer is still stuck relying on the issuer of the "real" shares to honour the blockhain sale, or on the seller not defaulting.
Edit: just to expound further on something I think most people miss: The primary innovation of Bitcoin was that it is provably scarce without relying on any legal framework. That's it, and it's pretty radical. If you start relying on the law to provide value, the blockhain idea tends to become extraneous.
