a lot of people do this and have been doing it for months. The item people are doing it with is an Animus. i've had several that are worth 1,000,000,000+ not that they need to be worth that much. a 60mill animus splits 15mill 4 ways.
If an easy way to make millions is what you are looking for, then you are all on the right track. Short of playing through the game long enough to unlock an animus and upgrade it to max for big profits there is another way:
As you play, look for ranged weapons that have 500+ projectile speed and a minimum of 25 upgrades. Try to find one that has a relatively low cost of upgrade, say no more than 2500 for the first level. You will need around 2.5 million to bankroll this little project. With weapon in hand and 2.5 million in the bank begin to upgrade the weapon pouring each stat boost into the projectile speed. The value of the weapon will improve gradually, but exponentially. As you approach the maximum level you should see the value start jumping into the millions. When its all said and done you should have a weapon that will sell for a hefty profit.