The question was not " if you would pay for a contract".
I am looking for beekeepers experienced in buying, selling contracts ONLY!

Thank You!
No offense intented but I don't believe you are looking for a beekeeper, what you are describing is a broker. When you use a broker you expect something in return, more so than a finders fee. A good broker arranges the contract and takes reasonable care in evaluating the buyers ability to pay and the sellers ability to deliver.
A beekeeper making an empty promise without assurances the grower will even use pollenator 'B' is selling the goodwill they purport to have with the grower. If 'A' has no intent on continuing to service the grower and does not have a long term contract you would have to question the practice, particularly if the sale is not fully disclosed to the grower. Pollenator 'B' would be better off to offer the grower a discount or signing bonus and build goodwill with the grower rather make payments to a beekeeper who is off to greener pastures anyway. This is more beneficial to the true players in the market (the pollenator and the grower) than the exiting player who has no intent of delivering value. Moreover, if 'A' is leaving for an undisclosed reason such as growers use of chemicals, poor payment history, etc. and this is not disclosed then 'A' is unethical.
If someone offers you a contract like this and lacks a long term contract with the grower I would recommend going straight to the grower and make your pitch. It's called competition. If the grower wants to go through a broker then that is his decision. If 'A' gets mad ask him what value he intended to deliver. If you are looking to sell pollenation relationships and do not have long term contracts I would recommend you first get permission from the grower and provide them some value such as evaluating the ability of who you sell your goodwill to, their ability to actually fulfil the service, references, etc.
When I was young I bought a 'contract' like this in another line of business without talking to the real buyer. The seller then wanted to 'break the news' after the fact so the buyer would ease into it - I considered it dishonest and was out 900 bucks for the lesson. I quickly learned that I could sell my own service just by asking - expecially after my reputation was established and I could hand out references to call. A true broker provides a service and has full disclosure and agreement with all parties. In real estate and business brokerage a brokers responsibility is defined by law and carries liability if not carried out. A casual beekeeper attempting to sell his relationships should be asked for his brokers credentials, what contract's he has with the grower, what guarantee's he provides.
In other industries this type of casual brokering of introductions is sometimes referred to as 'pimping'. However, there is precedent for this type of sale of contracts, if you bundle the contracts together and separate the pollenation fees into 3 traunches, then securitize each traunch and sell these to investors who pay 'A' for the right to extract payments from 'B' and then collatoralize these as debt obligations (known as a CDO) to insure the buyers of 'A's' contracts still get paid when 'B' goes broke from pesticide and colony collapse or if the grower goes broke from drought and cannot pay 'B'. This way 'A' delivers no service and gets paid. The investors get bailed out by the banks and hedge funds holding the CDO's who are then bailed out by the government. The grower and 'B' are out of business but have the warmth and comfort of knowing they own A's goodwill.