How to buy and sell on the same day without getting hit by "unsettled cash"?

If you sell T+3 then you would normally be able to buy (reinvest) T+3 or T+5 or T+10. There are some stocks that don't settle on time but that's a problem you cannot foretell. In the UK some bargains in Northern Rock didn't settle with the same settlement date and the same market marker and are still "open" although NR has been nationalised. normal. liquid stocks should clear or settle on the settlement date.