This would work if all sellers were selling the same goods. Such as in case of hourly labor. Incidentally, in these cases, the salary is known and is same for the laborers with the same skill set and responsibilities.
In case of salaried jobs, such as in Blizzard, the labor of each employee is unique and you cannot sensibly price it wholesale thus there are salary negotiations for both sides to come to an agreement. I personally do not see how knowing other people salaries is going to help you in a negotiation as this does not give you any additional leverage but if you think it will - go ahead and disclose your salary to your coworkers, see how it will go.
>"Shopping for the best offer" depends on knowing what offers were available to you and the others to begin with.
And who is preventing you from asking? I don't see how asking your coworker salary is going to help you to learn what offer you can get in another place without applying there.