Italian translation: contratto di compravendita di azioni (o quote)
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I also liked Teresa´s answer and reference, but I particularly liked the explanation of the difference between azioni and quote, which I´m sure I will find useful in the future. Thanks to everyone. 4 KudoZ points were awarded for this answer
Explanation: In Italian legalese I would translate your phrase as such. Just as patti parasociali refers to lateral agreements for voting stock and options betwween minority and majority stock holders. This, I believe, is the most Italian way of expressing your concept.