1. What constitutes "Joining" a mercantile venture? Is it being physically present (IE: A PC travelling with the caravan) or contributing capital or both or either?
For what purpose are you asking? I don't think I understand the question.
2. What happens if multiple PCs (Or one PC and his henches) attempt to search the same market for merchants?
Each adventurer may independently search the market for merchants. Each adventurer must pay the toll separately. Each merchant transacts with the adventurer who found them. The Reaction Rolls to persuade a merchant to buy or sell a particular type of merchandise use the CHA and proficiency of the adventurer who found them. (This prevents a character with CHA 18 and Bargaining from dominating the trade of an entire town.) There are approximately 20 times as many merchants total in any given market as the number listed, so in actual play a typical adventuring party is unlikely to "use up" all the merchants. That is, no one adventurer can corner more than 5% of the market personally.
THAT SAID, I've always felt these rules are incomplete. One thing I regret not putting in the rules is some sort of minimum threshold of cash or cargo capacity that a character has to own in order to qualify as a merchant in the eyes of those he would transact with.
As an optional rule, you could append the following to the ARBITRAGE TRADING section, following the sentence "This represents harbor fees paid to the harbormaster of a port, guild tolls at the city gate for caravans, etc."
"An adventurer is considered to have entered a market to buy or sell goods only if he arrives with either enough wealth and cargo capacity to be taken seriously by the merchants within the market. The Minimum Wealth and Cargo table, below, shows the minimum holdings of wealth and the minimum caravan or ship cargo capacity that an adventurer must hold to enter a market of each class. (Note that wealth can be in the form of inventory for sale)."
"If an adventurer does not meet these minimums, but still wishes to buy or sell in the market, he can do so, but many fewer merchants will be interested in transacting with him. When determining the number of merchants and loads of merchandise available to him, roll on the Market and Merchants table as if he were in a market of minimum size for the character's actual wealth and cargo capacity."
"EXAMPLE: Marcus arrives at Aura with a small sailing ship (10,000st cargo capacity) carrying a full cargo of common metals (100 loads, total value 20,000gp). He has an additional 25,000gp in gold coin. Aura is a Class I market, so normally an adventurer can transact with 2d6+2 merchants, interested in 6d8 loads each. However, Class I markets have a minimum wealth of 45,000gp and minimum cargo capacity of 20,000st. Marcus has the wealth but not the cargo capacity. Since Marcus only has a cargo capacity of 10,000st, he instead will roll on the Market and Merchants table as if he were in a Class II market. Had Marcus arrived with two small sailing ships he'd be able to exploit Aura's Class I market fully."
Market
Class
|
Minimum Wealth |
Minimum
Cargo
|
I |
45,000gp |
20,000st |
II |
15,000gp |
6,750st |
III |
6,750gp |
3,000st |
IV |
2,250gp |
1,000st |
V |
675gp |
300st |
VI |
270gp |
120st |