VCs are actually really important players here. They are so well connected and have worked on so many transactions, that they really help. I wonder how they would fit into an open marketplace.
Another side is that talking to a VC about selling is a serious conversation. It can affect things like likelihood of follow on funding, so opening one door can close another.
That VCs are thought of as a group of people who are well-connected, presumably to other well-connected people, does it make logical sense to think of them as a cartel?
No, because VCs don't team up with each other to try to restrict competition. That's the definition of a cartel. In fact, they regularly compete with each other by offering better deal terms (valuation, liquidation preference), or by bringing more to the table (counsel, networks, services).
There is no transparency in this market, and advice is especially lacking for those that have no VCs.