I wouldn't say these are hot tips, but common sense:
1. Know what a good price for the item would be new and used (say on ebay)
2. Know what the item is worth to YOU. It doesn't matter if it's a $500 vacuum cleaner if you aren't willing to pay more than $50 bucks for something that cleans your floor.
3. When you contact the person, be polite. If you intend to bargain, give them your offer right away, rather than waste their time
4. Don't commit to buying the item until you have seen and tested the item, unless you are willing to risk a broken item. People get upset if they expect you to buy the item, and then you don't.
5. If they can't meet your offer, consider whether you can go up a few dollars. Sometimes it's a better idea to just pay a few dollars more and get a good deal, rather than search for weeks to get a "screaming" deal
6. If the item is not as expected, and you would not be happy paying your original offer, you can make a lower offer, with explanation, but be prepared to walk away from the deal. Be friendly and kind throughout the meeting.
7. The longer your timeframe, the more likely you are to find a good deal. I've found some beautiful expensive items in good condition for low prices... after searching for years.
Good luck!