For safety reasons, a magical museum might well want to avoid putting actual items on display, especially since Illusion magic makes it trivial to create simulated substitutes crowds can safely interact with. This also keeps security budgets manageable, since even a magical museum probably doesn't have the kind of money to protect the arcane equivalent of a small nuclear arsenal.

So the museum researchers might be happy keeping items on indefinite loan, as long as they know the borrower and have guaranteed access when they need it. This way the mage owning the item is responsible for keeping it safe, and arcane historians--who definitely didn't spend all those years in magic academy just to play guard--don't have to.

As a bonus, this method gives the party a quirky but knowledgeable NPC contact who can give them clues or set up sidequests whenever you need.

When the party, the evil collector, and the museum rep are all together - there's a minor scuffle and then a fire "accidentally" gets started.

It can be as simple as someone tripping while holding a candle, and the candle landing on some drapes or carpets.

As DM, if you're good at what you do, you can play it up so the players think it actually WAS an accident that they need to deal with.

The collector will, naturally, start freaking out over their collection. And in their rage, the collector might blame and attack the museum rep (even if there's no way the rep could be at fault).

The players will then have many options:

• They put out the fire. If that happens, the collector may be so pleased the bulk of their collection is safe, they'll tell the players to hang onto the items... For now. And at what price?

• They run away as quickly as they can, saving the museum rep and escaping with the items. They talk, and the rep learns of their quest. The rep sees it as noble and just, and he allows them to borrow the artifacts until their mission is complete.

• In the chaos of the fire, they attack the collector, with the goal of taking as many of his other magic items as they can carry. They eventually succeed and escape... but they make a powerful enemy that day.

• Through no direct action of the players, the museum rep dies. The collector sees it as an opportunity to escape all blame, so he "allows" the party to escape with the artifacts... Only to immediately notify the watch of the murder and arson. Now, in addition to their quest, the party has a price on its head. And the watch wants to recover the items to give back to their "rightful" owner, the collector.

There are lots of creative ways to deal with the players, whether or not they take any of the obvious courses.

All you need is an easily-explained-in-game "accidental" fire. :)

I'm skipping the other comments because I don't want them to affect my thought process.

Your idea makes sense. Lots of IRL museums will put a replica on display, for reasons like value and risk of theft, risk of damage to the item even through regular light and air, or just because it's being restored or used for research at the moment.

I don't think they'd be able to create two items out of it, though. That's basically just cloning an item, though without the magic in the clone. That's not really something that is done.

I don't think a museum would be happy with an original artifact walking around. They'd want the original on hand for preservation and future research. Adventurers tend to lose, break, or sell things.

But you want the party to have the magic item, so I think you need to find a reason for them to change their minds and refuse to give up the item. You said it's a criminal collector, but I don't think garden-variety theft, smuggling, etc. makes it worth denying them, especially if they pay well for it. Can you make the collector evil somehow? Or at least make the artifacts useful for doing good, and putting them in a museum would mean people go hungry or something?

Let them attune to them and then donate the items, and it still works at a distance.

Later, they will lose an ability when it's stolen and attuned to someone else. Side quest!

Maybe the museum has the power to create a duplicate in exchange for the original. The original has value for research and display, and the duplicate has use for adventurers.

Maybe having the powerful magic items in the museum makes them a target, and the place gets attacked, and the players have to save it. As a reward and safeguard against future attacks, the museum researches a way to transfer the enchantment to a similar item? Sword for sword, gloves for gloves.

You could add additional Plot™ where after the items are collected there is something revealed that requires those 8 items that the museum is More interested in but is dangerous. As an offer for that item (which may not hold any actual magic but holds major historical value) they offer those 8 magic items to the adventurers as a payment from the museum to undertake the (dangerous?) quest.

You could easily make that historical item a representation of all 8 items as a way to satisfy the museums desire for those items as well.

The con to all this is that it would add one additional quest line to the story and might interfere with other Plot™ plans you have