Letters of Credit
Many is the player who has written to say "I don't know what the problem is, but I just can't seem to carry anything. I've put everything on my wagon and there isn't anything in my inventory except the sword I'm carrying and I still "can't carry anymore stuff". HELP!!!" It usually turns out that when they click the "Gold" button in their inventory, they find that they are carrying several thousand gold pieces. The solution to the problem is to exchange the gold pieces for a Letter of Credit.
A letter of credit is just like gold except it doesnt weigh anything (OK, it weighs 0.25kg and carrying a lot of them in your inventory can overburden you, but it's as close to nothing as you can get). You get it from the bank who will charge you 1% of the face value of the letter as a processing fee. In other words, a letter of credit in the amount of 100,000 will cost you 1,000. You must have the amount of the letter plus the processing fee on deposit before the letter will be issued. Taverns and ships are the only places that will not accept a letter of credit as payment. You must have cash to deal with these guys.
When you are selling items and you cannot carry the gold, the merchant will issue you a letter of credit instead. If you are making a lot of sales these will pile up in inventory and a lot of letters of credit weigh quite a bit. If this happens, take your letters of credit to the bank and deposit them and the bank will accept them as money, crediting your account accordingly.
A word of caution on selling items in batches. The value of the batch cannot exceed 2,999,999 GP. Once you hit the 3,000,000 mark, the shop keeper will offer you 1 GP for all of it. There is no apparently limit to the number of items you can sell in one batch, only the value of the items.