The guy was also selling a pair of panniers which he described as from an ST4 so the cans are almost certainly from the same bike. I'm pretty sure that the ST2 cans will fit anyway and if they don't I can always sell them again.
St4 and ST4S cans have a larger diameter pipe where they join the collector box. There is a very obvious flare of the pipe at the end. These cans appear to be the same diameter all the way to the end. Its difficult to see as its not a fantastic picture on ebay so i might be totally wrong!
You could make up an adapter if they are wrong though.
Here is a pic of my spare ST4 cans for comparison.
Check the ad again Paul He says they were on an ST4 but thinks ( incorrectly ) they'd fit a 2 or 3....The ends look exactly the same as my Sils I wouldn't worry Derek.