I would check the comparables out who have been used and see which one makes more sense. "Check for close to proximity, size and the most current sales" If you are buying in a declining area then you should trust the lower appraisal.
The appraiser who "Hit the value" does not want to kill the deal and then he/she would not receive anymore appraisal orders from the lender. I personaly do not like lenders I prefer a bank.
The seller agent should not have anything to say since he/she works for the seller not for you. "He/she probably knows an appraiser who would push the value. The agent gets more commission if the house sells higher. If the lender would agree to this then maybe you should change the lender.
The appraiser strictly is working for the lender to make sure that the value is there.
The seller agent would have a conflict because if the appraiser is hired by the seller agent then that appraiser is working for the seller agent not for you nor the bank.