You know, I keep hearing this from some people. Just how do you roll speculative trade anyway that you can exploit it that way?
The way I recall it working is you roll randomly on a table and get what is offered. Once per week. You can take it or leave it. You don't get to choose the thing with an actual value mod of -5 on this world and then jump to a world with a +5 avm for it and make a gazillion credits. You might get lucky but more likely you won't and you'll only make a good profit.
Profit in the LBB1-3 Speculative trade system (ignore the Merchant Prince system to earn the big bucks) is always a percentage of the base price, so cargos with a high cost per dTon will earn you most of your profits. [Lesser priced cargo is only useful when you are attempting to build up that inital working capital, since that is all that you can afford.]
So at every port roll for a speculative cargo and determine the purchase price based on the modifiers for that world and the random die roll. If the modified price comes out less than 100% of the base price, then you can be reasonably certain of making a profit on it. [As a personal preference, I restrict my trades to non-perishable goods worth more than 10,000 credits per Dton base price.]
For the sake of argument, assume that you eventually purchase one ton of a 100,000 cr per dTon cargo for 90,000 cr. (You can do better, but this is a fairly typical 'good deal'). Assume that there is a world seven parsecs away that has a combined +3 to the sale price due to trade modifiers (like Electronic parts on a Non-Industrial, Poor world) and a +3 Broker available.
You spend 14 weeks and 7000 credits transporting the cargo 7 parsecs (mean Ref says only J1 ships available) and 15,000 credits on a Broker-3 to sell it. You are rolling 2D6+6 for the sale price.
The WORST that you can do is a modified roll of 8 or 110% of the base price. Total costs are 112,000 credits for the 1 dTon cargo and the lowest sale price is 110,000 credits. You have a 1 in 36 chance of loosing 2000 credits on the deal. A more average roll would be 6 on 2d6 for a modified roll of 12 and 170% of base price. Total costs are 112,000 credits and the sale price is 170,000 credits - you earn 58,000 credits on the deal. A good roll would be 9+ (better than a 1 in 4 chance) for a modified roll of 15+ and 400% of base price. Total costs are 112,000 credits and the sale price is 400,000 credits - you earn 288,000 credits on the deal.
Imagine of you had purchased 10 dTons of the cargo instead of 1 dTon.
Imagine a 1 MCr per dton cargo.
Imagine another +1 modifier to the sale price.
Now to really abuse the system:
Some Refs FORCE the sale, but others allow you to say no and keep the goods. Keep trying to buy and sell cargo on each world and wait for 50% or less to buy and 200% or greater to sell. Some versions of the ‘Trader’ skill let’s you roll 1 of the 2D6 price dice in advance. A ‘Group’ could also apply Bribery as a skill to raise/lower prices.
Can you now see some potential to earn a LOT of money in a good deal. A couple of those good deals and the ship is paid off.