Speculative Trading Brokers and Guides

Steve H

I'm a little unclear on the effect of brokers and guides in speculative trading.
p.210 of core rules said:
Local Brokers and Guides
A trader can hire a local guide, to help him find a supplier, or a local broker to help him negotiate a deal. A local guide will have Broker skill equal to 1D-2. DM+1 can be added to this roll for every 5% of the total value of the trade that is given to the guide, to a maximum o DM+4. Black market guides require10% of the value or every DM+1.
To check my understanding of guides, let me give a worked example. None of my PCs have the Broker skill, so they decide to use a guide to find a supplier. They roll 1D-2. The die comes up 4 so their guide has broker skill 2. They decide to add a DM+2 onto this roll (do they need to do this before rolling?), costing them 10% on top of the final deal for the guide. So instead the guide has Broker skill 4. Rolling to find a supplier is now done with the guide's broker skill giving a DM+4. (Plus any other DMs. Presumably the guide's EDU or SOC mod should be added to the broker check too, but we don't know them) All good? Any errors?
Ok, what if they didn't bother with adding an optional DM to the roll that determines the guide's Broker skill? They're too stingy to cough up 5% or multiples thereof. Are the guide's services free to the players?

On to my question(s) about brokers. How seperate are these to the guides? I get the different roles. But are they being performed by the same NPC? Do we use the same broker skill as before? And again, does that mean they can choose to leave out the optional DM to roll to determine the Broker skill level, and not give the broker a cut (at least from the travellers end, the guide/broker might get a finders fee from the supplier for all we know.)