Is there a way for us to prevent the sale of the house until they have
moved the fence? For example, it seems that filing suit against the
current owners doesn't necessarily prevent the sale, and that the suit
would be pointless after the sale is complete.
You can't prevent the sale, but if you file suit to adjudicate the boundary dispute and file what is called a "lis pendens" giving notice that the suit is filed in the real estate records, the buyer will taken subject to the lawsuit and realistically, won't close in the first place.
The prospect of an impending sale is unlikely to be something that would cause the court to take expedited action or issue a temporary restraining order, because you can preserve your rights with a "lis pendens" which doesn't take a court order.
The absolute minimum amount of time in which you could get a court order on the merits without a temporary restraining order is perhaps three to four months, which is almost surely too long to prevent a sale of a house.
The suit isn't pointless after the sale is complete. You can still enforce the boundary against the new owner. And, often, a new owner, having no ego in the placement of the fence, might settle the case sooner than the old owner would have.
But, better practice in terms of neighbor relations would be to file suit first, so that the new buyer, if the buyer goes forwards with the sale, is aware of the defect. The old owner is also more likely to have evidence useful in the lawsuit you bring.
Short of filing suit, send a letter to the realtor and the neighboring property owner with a copy of the survey advising them of the problem. If you could somehow figure out who was handling the closing, you could tell them too. This will impose upon them a legal duty to advise the new buyer before closing, and if they fail to do so, they expose themselves to the risk of a fraud lawsuit from the new owner.
But, while this is cheaper, it also presents a risk that they won't warn the buyer who will then not have notice and the risk of being sued for fraud may cause the seller and the seller's realtor to resist your suit more aggressively and to be less prone to settle it.
If you wanted to be really aggressive, you could give the neighbor notice that the fence is over the line and that you will destroy it if the neighbor doesn't act, and then tear down the fence, which is strictly speaking within your rights if you can do so without a breach of the peace, because it is on your property. I wouldn't recommend this approach, however, as it could lead to violence or police involvement that depending on the policeman who isn't trained in real estate law, might get you arrested which is not good even if the charges are later dismissed.