[um_ya]

You could technically buy real shares and sell synthetic shares on the blockchain, harvesting the price difference.

This would generally have the same effect as the blockchain user buying the stock directly, but through an intermediary collecting premium.

[rebuilder]

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.

[ENGNR]

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

[SailingSperm]

These would also trade like other crypto - 24/7, 365.