If you limit your purchases to items whose value you are knowledgeable about you can secure decent bargains especially if you have any charisma and can negotiate.
To add to this go online and look up what items are going for used online. Don’t care what people are asking for them, that’s not indicative of the true value. If possible you want to know what the final bids in several auctions were.
Also when negotiating price remember they paid for it and need to make a profit on it. Any offer that doesn’t allow them to make money on a sale can be an insult.