Exchanges like Bitstamp are famous for demanding extra verification and some outrageously invasive information like tax documents if you trigger some unstated limit when you're supposed to be fully verified already. Others do it too.

I haven't experienced it myself because I've never sold enough but there have been enough cases I've read about.

If they want outrageous info then they should ask for it upfront before they have your balls in their hands. If I ever need to sell significant amounts I'll be using OTC operations that give you personalised service and leave you knowing exactly where you stand throughout. With an exchange you're just another ticket number.