i had no end of problems with the BB30 on my cannondale, i absolutly hated it.
it might be the sleeve that holds the BB, rather than the cranks - the BB cups tighthen into the sleave, which is in turn held in place in the frame by loctite.
on mine the water got into the space between the sleeve and the frame and as the water pulls in the grit, combined with a spray of lube, the loctite perishes and then the whole crankset will wobble and will feel like the crank is loose - on mine the entire assembly came out of the frame half way down whites level
the good news is you don't have to replace the BB and it doesn't damage the crank or frame, but it is a pain to fix as you need a special tool to get the sleeve out, but your LBS should be able to do it no problem - see if the seller will knock £50 to get it fixed - or better still get him to fix it before you buy
having said all that - without seeing the bike i dont know if that is the case, but it was with mine, so you might want to see if the LBs will have a look at the bike first