Dear Aranha,
Out there, there arent two distinct markets for SSF and stocks.
All orders are routed to Euronext.
You can specify that you actually buy the stock, in which case you will need the cash amount available in your account, or just buy on "SRD". In this later case, you do not actually purchase the stock, you will have to wait until the end of the month.Of course, you can sell anytime, just like any future contract.
"SRD" allows anyone, independently of his status or account size, to buy or short (with no uptick rule) with a 5:1 leverage.
As a matter of liquidity, those actually purchasing stocks are the institutionals and long term investors, everyone else uses the "SRD".
The volume of "SRD" orders is significantly higher than the "bought on cash" volume.
So I guess it will be the same with the US markets. This would significantly reduce the volume on the actual stocks (by 60% or so I guess).
I dont think this would cause any liquidity problems. Here, all orders type ("SRD" or cash) are routed to Euronext, so no arbitrage is necessary.
Hope this helps and sorry for the bad english
C2