We bought a home on 55 acres. A neighbor has since said that two people were buried there. The previous owner told him so. Shouldn't the seller have disclosed that at the time of purchase? What can I do?
Yes, the seller was almost surely obliged by county or state law to disclose the known presence of a "private cemetery," and he possibly deceived you by omission in order to streamline his sale.
In most U.S. counties, such burial plots must be mentioned in disclosure before a property's conveyance. And yes, you could sue the previous owner but would no doubt face stout attorney's fees, some of which you may be able to recoup if you prevailed. But a good real estate attorney will tell you it's a bad idea to go forward with a claim based on a perceived misrepresentation or nuisance. Either a case has legal merit or it doesn't.
So the fact finding is up to you. You don't say whether you know where the graves are or how long they've supposedly been there. On very old rural property that's been occupied more than 70 years or so -- particularly in the eastern U.S. -- it's not unusual to find old, unregistered private family cemeteries on land that has been rezoned residential. Such plots were a matter of practicality on family farms during America's settlement. But by the end of World War II, most states and counties in the U.S. required that all cemetery plots be registered.
In your case, it sounds like these mystery plots would legally constitute an "abandoned graveyard," whereupon you may have the right to have the remains interred in a more suitable repository following a public notification, assuming you can find them. Of course, you would have to do this at your own expense. As a side note, owners of private property on which graves are situated are also typically obliged to allow access by descendants and even maintain the area, though I expect this will not be an issue with you.
Whether this issue should be of grave concern to you is debatable. Certainly, if the thought of an old abandoned cemetery on your land haunts you, you may feel compelled to take action. But if you don't know for certain bodies are there, can't find any markers and are content to dismiss your neighbor's revelation as hearsay, doing nothing is likely your right.
As for future disclosure if you were to resell: That's up to you. If you have reason to believe that the graves were recently dug, well, that's an entirely different issue, of course!
Otherwise, consider the old axiom: Let the dead lie in peace.