Except in Uber's case where they are famous for keeping tight control on secondary sales.
http://fortune.com/2014/06/20/uber-plays-hardball-with-early...
http://fortune.com/2014/06/20/uber-plays-hardball-with-early...
I think the lockdown happened after that.
http://techcrunch.com/2011/02/27/jp-morgan-twitter-chris-sac...
He may have done the same for Uber, but I haven't heard anything about that at all.
"the Uber CEO got upset with Sacca for trying to repeat his Twitter move of buying up secondary shares in Uber from other initial investors."