Running the numbers for average rolls (assuming a 10 on 3d6) and no trade code DMs, any player with a Broker skill of 1 or better will usually make a small profit (15%). At that level, losses will happen fairly often, but players can always choose not to buy or sell if the rolls come out really low which will lessen the risk.
At a player DM of +6 it goes up to an average of 65% profit on each deal and the chance of taking a loss drops to almost nil. As always, paying attention to the trade code DMs for the current and destination worlds will skew the results in a positive direction no matter what the skill level of the broker.
Hiring a local broker makes sense if the players do not have a Traveller with a DM of +2 or more, otherwise it is not really going to make much difference. With the fees charged, a hired broker with a +5 DM nets about as much as a Traveller with a +3 DM and a hiring a +6 DM broker will add less than 1% more to the net profit.
These numbers sound reasonable to me, a skilled character (or NPC) can stand a good chance to make a fair profit on speculative trade but there will not be many huge windfall deals made. It makes hauling freight more competitive with trading, just like hauling passengers is now more profitable as well. In 1st edition, I would only carry passengers and freight the first few jumps to build up an investment fund for speculative trading, now all three are in a better balance and will likely be continued throughout the game.