I once bult a whole B&W G3 from components bought on eBay. Its the least reliable Mac I have ever encountered.

Having said that, I took a G4 iBook with a cracked screen today and swapped the screen with one which had had wine or something poured through the keyboard. Works a treat!

I tend to assume a seller with a broken Mac and good feedback will tell the truth when describing any issues or mishaps. This can be a great indication of what will and won't work together.
If the feedback is low or the description is vague, steer clear or at least don't pay too much.

Some words of warning: There are alot of iBooks with dead logic boards.