Alan, to answer your questions:
- The sellers don't have to be good traders. They always sell at nominal price.
- There is only a point in arbitrage if you can provide for the good to trade

. You can tell the administrator either to sell everything if he gets the money for it, or to keep at least n goods when selling to the market or to keep at least n goods when selling to the market
and loading to ships. As you want to automatically load all goods to the crayers/sellers, you can't use the latter version to keep the stock . Therefore, you must tell every automatic route to the town not to load the goods you want to do arbitrage with (this is nothing to tell the administrator, what you might have thought). For a reason I'll tell below, it is not wise to make the administrator to sell the arbitrage goods to the market himself. During arbitrage, the market will keep what it needs.
- If you keep 350 on hand, you'll get maximum profit in case the city grows to a size that it can cope with this amount.
- Indeed it is a timing problem - the office sells always at the same time once a day. The ship trades approximately four times a day, but four trades take slightly more than one day. Which gives the next point
- About synchronisation: In fact it turns out that you'll tie down twice amount of goods in two portions a and b.
1. crayers sell to market (a in market, b in store)
2. hulk buys from market (a in hulk, b in store)
3. crayer loads from store (a in hulk, b in crayers)
4. hulk loads to store (a in store, b in crayers)
5. - 8. the same with portions a and b exchanged.
so you have to start the hulk with buying from market in the time interval after the crayers sell and before they load from the store. There is one point where your competitors can disturb your scheme, and this is when there are quite a lot of cheap goods on the market. So you want to keep this state as short as possible. This means to start the hulk as quickly as possible after the crayers. Once you've started all three trade routes, they'll remain in that sync.