Genuinely couldn't say, but there's usually touts outside of the main stations, so Arsenal or Highbury and Islington.
When I've sold in the past, my mate has put them up on Facebook - that way you just need to transfer them to your membership number (assuming you have a membership..?) and there's no funny business.
I've just messaged him to ask what group/groups he has used, so hopefully have some better suggestions shortly.
