Used computers are hit and miss, unless the person who's sellng it is either a family member or a good friend. And sometimes that can make things even worse.
However, $100 may not be a bad price for that system if it's in good working shape. You may want to ask the seller if you can use it for a few days to see how it works.
If it seems to be in good shape, then try to get the price reduced, especially if the seller is anxious to get rid of it.
However, if he/she refuses to let you use it without buying, then walk away.
Also, if you're a gamer, then this system will leave you greatly disappointed, especially if you play the latest games.
It has only an anemic onboard graphics and no AGP slot(the latter being required for decent gaming performance).
PCI video cards are available, but they fall way behind AGP cards on performance.