Oh, sorry. I was scanning Active Topics when I read your post, I didn't really look at the title . . . I'll be more careful in the future.
As to how the system works.
For Speculative Trading, the basics are:
1-Roll on the "Locating and Selling Spec. Cargo" Table on page 358. Use a PC's Broker (may be rolled untrained) skill vs. the DC shown based on the Starport Type.
2-If the Broker skill check is successful, roll the dice shown to determine the number of cargo lots available.
3-Roll on the Speculative Trade Goods Table on page 360 (applying the Dice Mods found on column 1 of page 359 under "Buying Goods" to the percentile roll).
4-Roll the dice shown under the QTY column, and this is the dTons size of the lot (except where brackets are shown next to the dice, it's stating a unit quantity, and the Base Price is per Unit, not per dTon, although the brackets specify what each unit takes up in dTons).
5-The value under Base Price is the per dTon (or per unit) cost to buy or sell. If buying, apply the Dice Mods under "Purchase Modifiers" to the Actual Value Table roll, and if selling, use the "Resale Modifier" values.
6-NPC Brokers or PCs with Broker Skill may attempt to modifier the Actual Value Table roll.
7-The "Brokers" table on page 359 is wrong, as shown by the v6 Errata. It should read +/- 1 through +/- 4. Minus values are applied during purchase, plus values during sale, and the broker gets a fee based on the percentage of the sale as shown on the table.
8-The "Character Brokers" table works the same, except that the PC must make a Broker (or P/Admin at -2) skill check to see if they can get a AVT bonus. The PC picks the bonus he/she wishes to attempt, and then makes the skill check. If successful, they get the bonus they tried for, and if not successful, they get no bonus. The table shown here is also wrong, and should show +/- instead of just + (PCs can choose to push price up or down like NPC Brokers, but they have to choose ahead of time, of course).
10-Roll 3d6. Add bonuses for broker (NPC or PC) and the modifiers for Purchase or Sale as shown on the Speculative Trade Goods table on page 360.
11-Take that number, look up the percentage modifier on the Actual Value Table. This percentage is applied to the Base Price from the Speculative Trade Goods table.
12-Multiply the dTons (or units) in the lot by the Modified Price for the lot. That is the cost to buy or sell (depending on which side of the trade you've been going through).
Naturally, an NPC or PC Broker can have a big effect here, pushing price down during the purchase, and boosting price up during the sale. Having a Merchant around with a Market Analyst class feature can be very useful when trying to pin down what the eventual sale price of a particular cargo will be (lets you pick and choose to some extent).
Note, the rules list more extensive specifics. Just try running through them a few times, generating lots, determining quantities, using NPC Brokers (and the effect they have by taking their cut), etc.
