You might want to be prepared to put up some cash, the 7D is around £1000 new whilst the 5DMII is around £1500 UK side when new, so I'd expect there to be a few 100 difference for the two in the second hand market. It's already a local deal only (since the ad states no posting) so just make sure you meet up somewhere open and public to check things out.
Personally I would not make the swap. The 5DMII is a fantastic camera body, great in low light, good video, good overall quality, but its AF is nothing like as good as the 7Ds and the wider angle of view is not something that directly benefits my primary interest areas. Wildlife and such similar subjects requires the good AF and is helped by the narrower angles of view. Furthermore I've a range of crop sensor only lenses that would have to be replaced (either sold and replaced or replaced without sale 0 since I still have a 400D).
I know that's a rather personal answer, but read the reasons why - consider if the wider angle of view is something you want; consider if you need cutting edge AF or if you really don't make much use of it at all in the 7D; consider the additional hidden costs in the change such as requiring new lenses for some focal lengths.
In the end you'll have to decide for yourself which of the two cameras features best match up to your needs. It's not unlikely that you might find you've excuses for both cameras in your setup and at that point you'll have to decide which interest areas are of most importance to you as well as which will be least affected by the change.