In the buy/sell menu, highlight any unwanted equipment. While holding the R2 button, repeatedly press X. This works only on the items in the equipment menu (the shield icon), not the items in the Bag icon. This works in any shop, including the little fast travel buy/sell menu. This glitch may have been patched, and does not affect trophies. Note: The item you are 'cheating' with will be sold and gone when you release R2.
Grind in any level for maximum experience and get the Elfstone Of Congeniality, which offers a 10% discount on items. Equip ten of them, totaling 100% off. You can now get anything from the store for around 1 silver coin. You can also sell your most expensive item for 30,000, then buy it back for 1 silver coin, then sell it again for 30,000. The best place to do this is in the Ettenmoors. The specific time to start grinding for the Elfsones Of Congeniality is just after a save point, when the character Andriel says "I see sunlight ahead." To be exact, the first two Orcs are archers that you will kill. Take out everything and loot the bodies, but do not continue past the save point of the wooden gates. Go back to the closest Fast Travel, and go to Rivendell and repeat the process.