Why couldn't they do the incredibly obvious thing of saying "Hey why don't all of us merchants running this same route get together and set a high price so we can make even more money. It's not like they can go anywhere else other than our group if we all cooperate."
Cartels are hard to enforce, especially under those circumstances. Why is this one group the only group capable if working the spice trade? Unless the suggestion is that this cartel encompasses all merchants in the Empire.
What keeps one merchant from deciding to undercut the others, just by a little? What keeps a completely different merchant or groups of merchants from deciding to get into the business and undercut them? What keeps the Roman state from getting so annoyed that they issue a maximum price or just raise the tolls?