Actually in a coop the unit is owned by the association and you get the rights to use it and I believe that is either defacto or dejure what gives them the right to decide who gets to occupy it.
<code> "In a cooperative says St. John, an attorney whose firm represents more than 600 condominium, cooperative, and homeowner associations, the building containing the residential units or apartments is owned by a 'cooperative housing corporation.'"</code>
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<code> "In a condominium, each unit owner owns an individual apartment in fee simple. In addition, the buyer owns an undivided interest in the common elements such as the exterior walls, roof, pool and other recreational areas." </code>
The co-op association legally owns the units, but the board and board-members don't directly own the unit being sold.
I was trying to point out that the board members don't have much economic incentive to approve a particular buyer - if the unit eventually sells for less, it doesn't directly harm the occupants on the board unless one of them is the seller.
http://www.bankrate.com/brm/news/real-estate/condos1.asp
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<code> "In a cooperative says St. John, an attorney whose firm represents more than 600 condominium, cooperative, and homeowner associations, the building containing the residential units or apartments is owned by a 'cooperative housing corporation.'"</code>
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<code> "In a condominium, each unit owner owns an individual apartment in fee simple. In addition, the buyer owns an undivided interest in the common elements such as the exterior walls, roof, pool and other recreational areas." </code>
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Edit: Any idea why my code tags don't work?