To begin with I'd like to remind you that what we are playing now is just the first act. Having money being very tight at the beginning of the game makes it all the more enjoyable when you actually start earning some cash.
Also, it is very thematically fitting for a prison island.
Seccondly we do not have the crating system implemented yet. Crafting is a very good way to get some early game money and will be available in the full game.
Also you are playing the game way to honorably if you cannot afford any skillbooks. There are several steps you can take if you really want more gold.
To begin with avoid paying merchants with gold. Pay with items. The reason for this is that when you kill a merchant they only drop a small part of their inventory, but they drop every item (excluding gold) that you ever traded to them.
So here is how to get rich in this alpha build of the game.
Select one vendor is the least usefull, sell everything you don't want to that vendor. When you have bought everything that vendor has for sale and dont "need" him/her anymore you can kill the vendor and get all the items back.
Now you can take all those items, plus what the vendor had and sell to another vendor. With this method you can buy every skillbook from every vendor on the island without even stealing any paintings.
If you do not want end up killing all vendors, go rob the keep. Get the teleport gloves and TP up by the raised ladder. Now you can sneak thoguh the keep and steal a boatload of paintings worth thousands.
Something worth noting is that the vendors seem to restock each time you level up. You can use this fact to your advantage once you have acumulated more valuable items than a vendor has goods for. Trade them for everything they have, level up, and then trade them some more stuff.
The method of "recycling" loot by killing vendors becomes much easier if you make "goodie bags" with a suitable amount of worth in them. Makes it much easier to only haveto deal with a few goodie bags in your inventory rather than sort a hundred items every time you kill a vendor.