Why not pot some up and warn purchasers it "does tend to self sow". That way you've covered yourself. Personally, I work on the principle if someone has a lot of a plant to sell, it must spread like wildfire!

There was a question on here a couple of weeks ago from someone asking where they could buy it! I find it very invasive here in west London, so I don't allow it to grow at all, I'm afraid.

it took me ages to get it to take and it now seeds around but it is easy to pull up. those that buy it from me do get told it can be a thug but it is easy to remove. it is a good doer as they say.

It is indeed a thug, but we have one which is white (and it is not C. malkensis either).

Thanks for the opinions guys. I think I'll do a few pots and give people a friendly warning. I've been doing a couple of carboots a year for the past few years, just selling off the excess, and I quite often have people come up to me and say things like "have you got anything that will grow in shade?", so this might be the thing for them. (In fact it grows both in shade and in full sun at the bottom of a wall for me).