Every owner has the right to select their own estate agent to sell their property when the time comes!
Some years ago, there was a growing trend for estate agents to offer “rewards” to a scheme, in order to be appointed by the scheme as the only agent who was allowed to sell properties in the scheme.
On the flip side, there was an equally bad trend for trustees to demand reward (for the scheme obviously), from agents who sold properties within their scheme.
Fortunately, the PPRA has now made it crystal clear that such practices are not permitted and that the scheme has no right to impose any agent on any owner, nor a right to demand payment of any form from an agent, following a successful sale.
Whilst a reasonable fee, in line with the work involved, may be charged to register an agent with the scheme, and to provide them with the scheme’s rules, and to provide them with a mechanism for access, such registration must be open to all registered estate agents.
There are some who argue that if you word your Conduct Rules appropriately, it is perfectly in order to restrict which agents may sell properties in a scheme. Such an argument does not hold water. You can and should have rules pertaining to allowable signage, procedures in respect of show days, restricted times for viewing etc, but that is where the authority of the scheme ends.
The CSOS will not approve any Conduct Rules that make any attempt to restrict which estate agents may or may not sell properties within a scheme. 
The above is not to be confused with clauses which require a percentage of the sale, or in some instances a percentage of the capital gain made by the owner, to be paid across to the scheme. These are charges levied on the owner and are unrelated to the agent who makes the sale. Such clauses would need to have existed from the outset, when the scheme was first proclaimed. They are almost impossible to introduce at a later stage, since they are unlikely to garner enough support from owners.
For those of you that are thinking of selling, remember that “buyers decide in the first eight seconds of seeing a home if they’re interested in buying it. Get out of your car, walk in their shoes, and see what they see within the first eight seconds.” – Barbara Corcoran