As far as prices go, really depends on both the seller and the buyer. Even the best items often get sold for free, while crappy items get sold for hundreds. A bunch of people, including myself, would never pay much (if anything at all, beyond a recommendation) for items that only grant prestige bonuses. Skill bonuses do add great value, though, but again, you need a buyer who actually has the gold you are looking for, if you want a big price, and one you can trust won't simply feel like you are being greedy and will arrest you right after the transaction (or even before).
