When I sold pups they were sold for 50 dollars or 75 dollars with no strings attached because I know some handlers can ruin a pup and I didn't want to fool with having to replace a dog or a pup.
In the mtn cur breed I have seen top notch dogs hunt for their current master and are as good as any I have seen but wouldn't hunt for anyone else for a month or even longer. My friend bought a well started dog that wouldn't hunt for him for about three months. We knew he was a very good strike dog and then one day he turned on and was back to his normal self.
The best way to buy a dog is to hunt with it 3 or 4 times without any other dogs and with the handler (seller) present and then negotiate the price and details. The next thing to do is to have the dog learn to trust you (buyer) so he will hunt at his best for you asap.
Like a previous post, someone mentioned that some dogs can be ruined by the new owner and that is another reason to charge more on a trial basis and I agree.
Of course there are dogs that will hunt for anyone right away if they have it in them to do so.
What really gets to me is the people who sell a sorry dog. The usual excuses are that the dog needs more time, needs to be hunted more often etc. etc..
If the dog is 1.5 years old or older a it doesn't hunt it is a cull.... It doen't need more hunting time...
For me a good hunting dog needs to show a lot of potential at 1 year of age with limited training/experience or I wouldn't buy it, and trashy, ran a deer for two hours on a pitch black moonless night by himself is a plus if you ask me. That just might be all the trial I need...
If I owned a dog that didn't show much potential at a year of age I would tell the potential new owner exactly what I thought and the dog would be given to him for free because if I didn't want it and I expect the other party should get the same treatment. It doesn't mean the dog won't hunt at a later date but I wouldn't keep it and it wouldn't be for sale. It would be free.