Some points (IANAL):
* How is it going to be transported? Squashing it into airplane luggage is probably not a good idea, neither is sending it via a parcel.
* What happens if something happens to it during transport? Generally works of art are insured before they are being transported. Do you know insurers though? Before you insure it, you need to determine the value.
* For very important works of art, countries might put restrictions onto exports. Not sure if this includes the US, but customs are certainly an important question.
* You might get sued by the rightful owners for wrong handling... ideally you want an agreement that releases you from any requirements to pay for damages to the object.
* Ideally the insurance, transport, etc is paid by the rightful owners, which would be good to have in a contract. How do you enforce that they really pay? Ideally, the owners would take care about the insurer and transportation directly, so that it's them getting the bills instead of you getting them and then having to get the owners to pay them.