Found Treasure In A New House And Need To Know Where To Go

The devil is in the details, folks. Note that the OP stated the collection was found HIDDEN in the basement. The truth? An unfortunate choice of words? Whatever.

That alone could mean that the seller was unaware of its presence. Perhaps someone in the seller's family or a housemate placed it there some time before they died/moved out/whatever without telling the seller. At least have the courtesy to ask before disposing of it.

Perhaps it was there when the seller bought the place. In that case, no foul; it belongs to the new buyer.

Most real estate transactions require the seller to provide a
property that is "broom clean" prior to closing!

The buyers will normally be offered a walk through prior to closing.
The buyer must accept the property as is, or ask for credits for extra disposal and the cleaning out of sometimes more than many are willing to do.

Bank owned and some estate sales will spell out that it is the buyers responsibility to accept the property as is. After the closing, all of the property on that property is conveyed to the buyer.

Fertile imaginations at work!

HIDDEN - ah yes, and what was the seller REALLY trying to hide? What crime have we chanced upon? Remnants of a STOLEN collection? Only the Shadow knows, so stay tuned for the next segment of our unending story.

Oh the things we cannot know!

But what we DO know is that the law provides for title to pass to the buyer at the point of closing on the property unless an alternative agreement is reached prior to closing. For us that should be the end of the story - without dumping anything on the new owner.

Once the final walkthrough is completed and the sale is finalized and title transferred, ownership of the contents of the property pass to the new owner, as do any unrecognized problems/defects.

The onus is on the seller to make sure they have removed all goods they wish to retain prior to the buyer taking possession of the property, barring other arrangements to the contrary.

Structural problems/defects work the same way: the onus then is on the BUYER to make sure that a proper home inspection was done and all problems addressed or remedied prior to closing. Once the buyer takes possession, if they discover an unforeseen issue, too bad (home warranties notwithstanding).

In short: no takebacksies.

Congratulations on your find! What a nice house warming gift. Have you ever considered starting a stamp collection? Maybe someone in your family might be interested.