20,000 shutter clicks is absolutely nothing, completely ignorable. For what I do in sports, it's easy to shoot 5,000 in a day. I've shot as many as 12,000 in a day. The back up to my back up to my back up Canon 7D already has 36,000 photos on it and I consider it to be brand new. In fact, I've only had one camera shutter fail so far, and that was a Nikon D7000 after 136,000 actuations (it was completely replaced for free under warranty).
Pro bodies like this one have never had a shutter failure for me, but I'm probably lucky because they have hundreds of thousands of shots, and they definitely DO fail (more often for people who don't have backups, it seems to me, however!).
But just 20K is completely irrelevant. Don't return it. Just enjoy it.
As far as lying, the other posters do have a valid point there. A lie is a lie, and maybe you can't trust the seller to be clear about other details. But it is possible that those extra exposures were actually put on the camera AFTER the sale began. For example, he used it for a wedding while the auction was going on... 3,000 actuations... for track and field in Moscow.... 25,000 more actuations, etc.
But if there is anything wrong with the camera worth returning it, it has to be something else, not the actuations.