For countless eons, Bartiok's people cherish conquests and possessions. They believe that all things, including other beings, could be "owned".
From birth, Bartiok was taught that he was the "property" of his father, to be used, traded, or outright sold at the whim of the owner. The only way he could be considered a "person" in that society would be to kill his or her owner, whereby they would own all of that person's possessions (including any other beings). It was considered a great honor for a son to kill his father in that society and thus inherit all of his possessions. Unfortunately, in Bartiok's case, his older brother had that honor, and then he was sold to another owner. Bartiok eventually took the life of his owner and took possession of all that house provided, but he was fixated on the conquests he could not make.
Since his people have extensive lifespans (approximately ten thousand years as measured on Earth), Bartiok witnessed several changes in societal thinking, including a global civil war over their long-established concept of ownership. Sadly, he found himself to be on the losing side of that struggle, and thus he decided it would be best that he took what things he did own and leave his world for places that would share his love of ownership.
In the vast reaches of space, Bartiok found his niche, as the rich and powerful in the cosmos were always looking for new and unique things to own, be it art, ships, weapons, planets, or other beings. The problem for them has always been of acquiring those things. Taking action themselves would often jeopardize their reputations, especially when the things they desired were considered "endangered" or near-extinction.
Bartiok offered his services to those rich and powerful beings as the means to their end. He would be the public villain for them; the collector that would get them what they will pay anything to have. In exchange, they would give him credibility in the intergalactic community, and he reserved the right to claim ownership of one item from any assignment. His sponsors were reluctant to agree to his terms, but eventually did because he was willing to give them what they long desried.
As he built his reputation, Bartiok acquired a sizable fortune, which was enough to furnish his ship, hire a crew, and even to eventually own several worlds to store his own collection.
There were some things that Bartiok considered to be not worth the effort in collecting. One such item was a Kheldian. The famed cosmic travellers were sought-after by the mysterious Libra for reasons that have yet to be explained. However, an agent of Libra later gave him something even rarer to go after: the Guardian Exiles.
Seven years later, his "advance agent" Nox arrived in on Earth.
(Note: Further information is restricted by Vanguard.)
In an alternate future, Bartiok frequently threatened the surviving humans after Earth was rendered inhospitable and they were considered an endangered species. Being transported to new worlds in ion-powered sleeper ships, humans were easy victims of pirates and collectors.
However, Bartiok's attempts to "collect" these humans were thwarted by Galatea Future, the Last Guardian of Earth. Eventually she would defeat Bartiok and he would again become "owned", this time by a prison planet.
According to writer/editor David 2, Bartiok was inspired by several characters including Darkseid, Thanos, Larfleze, and the Necromongers from the Riddick series.