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. :)