First off, I think "buy" is an unfortunate choice of words when talking about a race that enslaves people...
A few episodes later its 'revealed' that actually its the women who are in charge, controlling men with their pheromones. If thats the case, why do they allow themselves to be sold and why are they so prized a century later, if the 'owner' will actually be under their control?
Protective camouflage. It's safer to be the hidden power behind the throne than to be the one overtly sitting on it. That way nobody shoots at you, they just shoot at your replaceable puppet.
Besides, there is historical precedent in real life. Our perception of slavery is based largely on how it was practiced in American plantations, which was just about as awful as slavery has ever been; but slavery is an institution that's taken many forms in many different societies. In the Islamic world, slaves were routinely used as administrators and warriors, given considerable power even while technically being owned by someone else. Slave armies would go out and conquer entire nations, becoming their rulers and starting hereditary dynasties while still effectively being the property of the ruler back in the home country. (In the Ottoman Empire, all subjects, even the rulers, were considered slaves to the state itself.)
There's also precedent for women wielding considerable political power even while seemingly being subordinate. A prime example is
Hurrem, initially a harem slave of Suleiman the Magnificent, who used her wiles to influence the sultan and gain status for herself and her sons, as well as becoming his advisor on matters of state and, according to some accounts, effectively running the Ottoman Empire from the harem. There are lots of ways that women and other subaltern groups have managed to wield informal power throughout history. Women in particular have wielded considerable influence through their control of family matters and marital choices, shaping alliances between clans and nations through the marriages they arranged or participated in; or by using seduction to manipulate or compromise male leaders; or through their role as mothers, guiding and influencing their sons' beliefs and decisions.
So to someone who's studied women's history, ENT's revelation that the role of Orion women is more complex than we thought, that it's not merely a simplistic masculine power fantasy about "animal women," actually makes a lot of sense. If they're really as intensely seductive as they were established to be in TOS, that's a source of power for them, and it makes perfect sense that they'd use that power for more than just being fetish objects for men.
Add to this the fact that the Orion Syndicate appears to be the name of their government/empire/territory. I always thought the the Syndicate was more like a large criminal organisation, as when it was on DS9, there wasn't an Orion to be seen.
I don't see why it can't be a criminal organization in ENT too. Mobs have territories, "turf" that they control and defend. If that part of space was otherwise lawless in the 22nd century, the Syndicate might've been the closest thing it had to a government. The rise of the Federation may have diminished the Orions' power and prominence, so the Syndicate would've needed to evolve, becoming more of an underground group, maybe merging with other species' criminal organizations or broadening its recruitment to survive. A lot can change in two centuries.