all in eol if you have basic level 2 info you can see these types of trades going through so relatively easy to spot.
As in a rollover example, if i held 10,000 shares on a T25 settlement, rather than pay for them until i have the cash available I might get the opportunity through the likes of TD Waterhouse or some broker who will give me a further 20/25 days credit, hence they have to be settled (sold) and they immediately buy them back at a smaller premium than the normal spread - hence the term rollover. You can't do it indefinitely.