"Between 1964 and 1989 some 33,755 political prisoners and 250,000 of their relatives were sold to West Germany, for a sum totalling 3.5bn Deutschmarks," says historian and author, Andreas Apelt.

"Both sides had an interest in the business - the GDR because it needed Western currency and the West because it wanted to save people from the inhumane prisons of the GDR."

Prisoners were also traded for commodities such as coffee, copper and oil.

However, neither side wanted the public to find out - the GDR because it didn't want to appear weak and West Germany because it didn't want to be seen supporting the communist regime.

So the operation remained clandestine - people were traded in darkened nooks of the underground railway, the U-Bahn, or sent across the border in buses with revolving license plates. The number plates would switch at the checkpoints, so as not to arouse suspicion on the other side.

Here's an interview with a woman who was sold, along with her mother and father.

http://www.bbc.co.uk/news/magazine-29889706#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa

I'm fascinated by her comment that "…if I had stayed, then I would have made a good life over there. People were well looked after and I agreed with the principles of the state - I still do - just not all the spying and oppression."

Spying and oppression is not a bug, it's a feature. Communism can't work without it.