Craigslist is great, but it also sucks....

Problem is, the cost of using CL is so low, people don't have much invested in their CL experience- it's just too easy for both buyers and sellers to be complete flakes and fear no consequences due to the anonymous nature of CL.
I usually try to make sure the seller knows I'm serious, and that I know what I'm looking for. Especially if it's a screaming great deal- you have to make it as easy as possible for the seller to get the thing sold. I try to be specific, mentioning that I work nearby or live within a 15 minute drive of them, etc etc. I also mention that I have the means to pick up whatever they're selling (pickup truck or car trailer, depending on the item)- and that I will bring CASH and be there when I say I will.
I usually include my phone number at the end of the email- some people just LOVE to use the phone vs. email.
Not much else I can offer, but be persistent and you will have success!!