The barter scripts in tDU were supposed to handle some of that trade. Such things are hard to handle with normal stores because the relative prices have to change, what's cheap in the county is expensive in town, and vice versa. The big problem with tDU, though, is that the local economy was supposed to be broken, supposed to be dysfunctional. Starting back with LotD gives me a chance to show a functional colonial economy, and then slowly have the war wreak havoc upon it.
A neat little thing I've already scripted is that the pawn shop in Port Royal extends you a line of credit, starting with 1000gp and then slowly rising as you make payments. Every time you walk into the shop (after a day or two) the shopkeeper demands a payment (like 10% of your balance) before he'll show you his shop. Go too long without paying and the Gendarmes come after you.
The hard part is, though, that the only way to get actual gp is to sell country produce to the town merchants (or turn bounty-hunter and chase down fugitive gnomes). Many items the PC can hunt or gather for themselves, but others will come from friends and trading partners. Dei'dre, for example, will give you a bottle of Danaan Stout every so often, but if you don't ever give her anything, she'll cut you off.