Put your most expensive item in the junk section in your inventory. Find a 
merchant, enter the "Junk" section in your inventory, highlight the expensive 
item, then press Triangle followed by X very quickly. If done correctly, your 
item will be sold at twice its value. It may require a few attempts. Buy it 
back, and repeat the process as many times as desired. It is best if you use two 
fingers, with one on Triangle and another on X so you can press them almost 
simultaneously. It is recommended you either do this with a merchant that you 
have not sold any junk to before, or buy back all the junk you previously sold. 
This speeds up the process enough so that you can double the gold as fast as 
possible. Additionally, if the merchant has an item more expensive than the one 
you are selling, buy it, and use that instead to increase the amount of gold you 
get. If you have the Blood Dragon Plate, this can be easily done with the first 
merchant.