|"Hey kid, wanna buy some sheep?"|
I use these by rolling on the d100 table when the party encounters a merchant ship (most of these are either common galleys (150 tons of cargo space) or trading cogs (300 tons of cargo space)) (ship stats here) to figure out what goods they have aboard. If you want to really systematize this you could roll a d100% to determine how full the trader's cargo hold is and roll on the goods table and the quantity per rarity table to fill up their hold to their current level, but I have found that that level of detail is often not super important.
I have found this table to be pretty useful for determining the primary industries of islands or even the domains that Gods and Spirits are interested in. So that's cool.
Random Good Table:
Livestock Head to Ton Conversion Rates
Quantity Per Rarity Rates
I also wrote a little calculator using Excel's random number generator to simulate a variable price when selling goods in market. The math is pretty simple, and it seems to work well:
Selling Price= Common Price*(1+x)
Where: x=(random value between -25% and +25%)/y
Where: y=1(if very rare), 2(if rare), 3(if uncommon), 4(if common)[I especially like that the rarity controls how big the price variation is. Common goods are only ever -6.25% to +6.25% above or below the common market price, they are more stable good because they are common. Rarer goods can have bigger swings in price, like in real life! Economics simulation tirade over]
Or you can just say fuck it, Connor you wrote a god damned math for D&D what the fuck, give me the math in a little calculator for fucks sake. Okay.