Each time one buys or sells units of an ETF trading or brokerage fees are paid. (tsinetwork.ca)
If you hold shares indirectly through a managed fund you can sell the shares by selling your units in the managed fund. (moneysmart.gov.au)
The seller is not under any obligation to sell the unit for any less than what it is listed for and even at full price they still can turn down offers. (wellingtonfloridashortsales.com)