If a store has an ad for a sale item and you go to the store and the only item they have is a display item can we make them sell it to us?

Yeah they will sell it and then you say Hey since this a floor model can I get like 5% or 10% off the price. Sometimes they will give you discount sometimes they won't.

You CAN'T MAKE them sell it to you at all.
You can ASK if they would sell it to you, but that's it.

Yes you can ask and usually there's no problem.

However if it is made especially for a promotional display, they can refuse ,because they will be getting more stock in, but not a new promotional display, so will need to keep hold of this one.

Generally whether a store will sell the demo or floor model depends on whether they will be getting additional stock on the item. But the store can make the decision, not the purchaser.

Ask for a rain check if the item is sold out. Most display items of mechanical/electrical products have been heavily used during store hours. Asking for a discount on a better quality product is usually a win-win for both you and the retailer.