Do you have any tips for buying a second-hand PC?

I've never bought a PC second-hand and I've never sold any of my PCs either, so I'm not all that qualified to answer this question.  However, what I can do is list what I'd look for if I were to buy a second-hand PC.

Be careful!  It's [...]

Burn a lot of CDs or DVDs?  Do some go wrong on you?  You need to read "Avoid the top five disc-burning mistake s" over on Computerworld.
Here's a taster of the tips:

You didn't verify
It's the wrong media
You're going too fast
The firmware's gone soft
You're cruisin' for a bruisin'

Sound tips!
I've three additional tips to offer:

Don't handle discs too [...]

Is it OK to use rechargeable NiMH batteries in cordless keyboards and mice?

As a rule, yes, you should be fine using rechargeable NiMH (Nickel Metal Hydride) batteries in cordless keyboards and mice.  Just don't bother trying to use the older and weaker NiCd (Nickel Cadmium) type - they won't last long at all.
I tend to [...]

Here's one, the author of this scam email.  Just because someone says they have honorable intentions doesn't mean they do.

Dear Friend, I am Mr. JOURAN JAWAD a merchant in Dubai in the  Middle East.(German Citizen) Now based in United Kingdom .I have been diagnosed with Cancer which was discovered very late, due to my laxity [...]

Gotta geek that you are shopping for?  Want some holiday gift ideas?  Then you've come to the right place!  Over the next few weeks I plan of offering a number of gift ideas for geeks.
 Here's the second gift - Infrared thermometer .  Dead handy for testing how effective the PC cooling system is and spotting hot [...]

Pre-order the Optimus 103 key OLED keyboard  for $1,200.  Yep, you read that right, $1,200.  These will ship around shipping May 2007.
Don't all rush to order one at once!
And this is for a monochrome OLED version ... here's what the makers have to say about the color version:

Optimus-103 will use black and white displays (B&W is [...]