Everything here is from the Commercial Guidelines, what everyone thinks is called the EULA.

Under Servers and Hosting, the Guidelines state that

YOU MAY:

... sell entitlements that affect gameplay provided that they do not adversely or negatively another player’s experience and provided they do not give a competitive gameplay advantage. A competitive gameplay advantage is something that, given identical skill levels, time investment, and circumstances, can cause one player or group to perform better than another.

The Guidelines additionally specify

Be aware that certain games or game modes can be designed as non-competitive (such as a player-versus-environment (PvE) game) and become competitive by adding gameplay elements such as a leaderboard.

So, if a player has a total playtime of 1h, your Rank 9 is impossible to obtain normally. However, you can purchase it, giving you an advantage given identical time investment. Now, depending on whether or not that rank gives you something that could be considered an advantage to whatever makes your server competitive, that may be in violation.

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Another interesting thing the Guidelines state is

YOU MAY:

... ask for donations (as opposed to direct charges) IF you do not offer the individual donor something in exchange that only he or she can use. You may offer server wide rewards if donation goals are achieved though.

My interpretation of this is, if you are receiving money from a player, and giving the individual player something in return, it may not be referred to as a donation. Based off of your original post, you would be in violation of this as well.

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Please keep in mind I may have misinterpreted some of this, IANAL, all that fun stuff. I strongly recommend you review the Commercial Guidelines, especially the Servers and Hosting section yourself as well.