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I beloeve that most listing agent will not do this as this is violate law and contract with seller. Most institute sellers have their own inside people to decide which offer to be accepted. Not the listing agent. and if the listing hide an offer not to present to seller, it violate law and National Association of Realtor code of ethic. The result can lost license. As final sales price is published on MLS, the buyer who make higher offer and not get the property may ask the seller direct after case close. It is easy to be found.
When a property has no offer, the listing agent should glad to help you make an offer and tell you the bottom line as most institute seller use the same stratige to sell. Most experienced agents know that "bottom line".
And if you are smart and know how to, I suggest you get a real estate license. I sponse many real estate agent who only service his/her self and very close friends.
I am a real estate broker. Sorry to type in English. Type Chinese is too slow.