I read a few posts here about legal ways to make money. Yet I didn’t see anyone mentioning this method.
All you need to do is haggle with merchants. It’s simple, but first you have to put in a little bit of work to raise your reputation in town (do quests, help people). This will make people sell cheaper and pay more. Also, you will need high speech skill. So after some time there will come a point, when you can buy from one merchant and sell it with a much higher price to another. Cheapest merchants I found were Bath owners. And people who pay most are actual Traders. (Peshek pays the most, but he doesn’t count since he is part of crime organisation)
Also, remember that the more you sell, the more money merchant has next time.
That’s how much I paid:
And that’s for how much I can sell it:
(but of course in reality I sold it to Peshek for 300+)
Also, don’t be too greedy or you might meet his fate 
