I've successfully acquired and used a couple of used laptops and several used desktops over the last few years. However, I don't recommend buying used unless you have the time, patience and, possibly, the resources you may need to get it running right.
With that said, if you still plan on buying used, I would offer the following advice I would buy locally so that you can at least power it on and test-drive it before you pay for it. Try searching Craigslist for your area as one resource. I've also had reasonable luck at garage sales (but lousy luck at flea markets!).
As far as cloning the hard drive in case you have to recover the system at some point, i personally wouldn't be comfortable with that. You could be cloning a problem without even knowing it. I'm not opposed to making and using an image of my drive. I just wouldn't trust the drive in a used system unless I'm the one that performed the last clean install.
To give you an example of success working with used equipment, the laptop I'm using to post this is a used Dell Dimension D600 which was being thrown away by a small doctor's office. His out-sourced IT service told him the hard drive was "shot" and the laptop was too old to spend more money on. Having a little bit of experience with Dell machines, I knew that their partition system, specifically their utility partition, had a bad habit of becomming corrupted. This system would power on and allow me to run the internal diagnostics, but would not load Windows. I took the unit home and pulled the hard drive to get the make and model. Using my desktop (which was also used and free!), I downloaded the manufacturer's free hard drive diagnostics from their website and burned it to a disc. The HD passed all tests. The next day I placed a call Dell Tech Support and explained that I had acquired a used system, but the previous owner didn't have the recovery discs. They promptly shipped me, free of charge, a new set of discs for Windows XP Pro, the OS that came with the system. (Other companies like Compaq/HP system, will charge about $30 for the original OS with all bundled software.) A note of caution, Dell does not provide the bundled package that originally came with the system. They only provide a clean copy of the OS. For me, that was all I wanted. I performed a clean install of Windows XP Pro, installed my anti-malware applications and third-party firewall and ran all the necessary updates.
Buying used is risky. You could do all of this work, spend some extra money on improvements and get the system running just like you want only to face some expensive or catastrophic failure the next day. For me, it's a hobby. I enjoy scavenging. And, if I lose one here and there, I'll scavenge it for parts.
Anyway, good luck! Hope this post helped a little.
Edited by StartingOver, 25 September 2010 - 11:54 PM.