Ten ideas off the top of my head.
1) Attempt to sell it to the forces of Chaos as part of an elaborate plan to destroy a Space Marine chapter. A Chaos Marine will wear the power armour and you will report them as a "survivor" lost in the Koronus Expanse (or your particular region of space). Once the chapter arrives to reclaim their lost brother, everything goes horribly wrong.
2) Fill it with explosives and present it as a gift to an enemy of your Dynasty as a "Peace Offering" between your two houses. Do not deliver it in person.
3) Hide it in the holdings of one of your enemies and alert the Space Marine chapter that it belongs to that your enemy has captured and killed one of their Space Marines, and you have managed to recover this information for them.
4) Depending on the chapter sell it to the Adeptus Mechanicus for study of this ancient suit of Power Armour, or to the Inquisition so that they make inspect it for heresy or gain a bargaining chip over the Chapter in question.
5) Take a series of pictures of you with your new Space Marine best friend. Pose the armour into a number of amusing positions and then hand them out to people to prove your piousness.
6) Throw the armour into a star. This probably won't accomplish anything, but it will cement your reputation as a man of unpredictable character.
7) Release propoganda videos starring this Space Marine chapter extolling future trading partners of the importance of a rare and obscure mineral that they have never heard of before. Show up in the system selling this mineral.
8) Fake your own death at the hands of the Space Marine armour in order to convince Chaos Raiders/Dark Eldar that you are a man not to be trusted inside the Imperium. For good measure slaughter a couple thousand of your own crew to add to the verisimilitude of this deception.
9) Hide your Warrant of Trade in it. Reveal to the Ecclesiarchy that your Warrant of Trade is in fact encased in a suit of Space Marine Power Armour. Look totally pious.
10) Undergo Heretek surgery to implant yourself with gene seed grown organs that were probably not tainted and corrupt so that you yourself can wear the armour. Have everything go well forever from this action.