From my memory I assume, that you can have several selling operations during your turn of the stockround and one buyingoperation.
That assumption is really the heart of my question. I don't see any specific mention of there being several different 'selling operations' during your turn. Nor does it explicitly say that all of your sales happen simultaneously (although there is mention that all sales must preceed your one buying operation in a given turn).
For reference, here's what Section 5.2 Selling Stock says...
A player may sell any number of stock shares on his or her turn, subject to the limits below. The player receives the current stock price for each share sold, places the stock certificates in the Stock Market and adjusts the stock price for each Corporation in which share(s) were sold downwards one box (even if the player sold more than one stock share of a given Corporation during that stock sale).