Offer people stupidly low prices, a lot of the time people really need the money when they start selling there personal belongings on craigs list.
This should always be taken advantage of, I just got a trial bike that was listed for 675$ for 300$ cash.
Also try lying - It works, say a friend of yours or your son (I find that if you use your own kids they expect you to show up with them) fish tank got broke and your trying to get them a good deal, because there on low income, yada yada.
"Crap Ware" is one I use alot, tell them a little bit about how outdated there stuff is, You can see when it sets in set in in there face. Offer them hella low. Don't know how well this would work with fish tanks, maybe say "looks like it will have to be resealed soon, thats gonna cost a chunk of change."
Although sometimes, I'm looking at one online right now for way less than that i'll give ya "X" works too. Some people just post higher than they want incase someone wants to pay it.
I used to post the same item twice one high price, one normal price. It can help your lower item sell fast - gotta get the good deal before its gone right? Or sometimes people will think "Ooo I want the better one, it costs more it must be better."
Yes, i work very hard at hardly working = P
You might not get it the first or second try, You have to fish for the joy of the sport, not the catch.