Here are some tips you can consider before deciding to buy a used laptop:

1) Age of laptop. Any laptop older than 3 years would likely not be able to support many new softwares. However, if you were to use it plainly for surfing and simple word processing, you can still consider paying about USD100-200 if the laptop is in good condition.

2) Condition of laptop. If the laptop had suffered a fall before, or had be physically damaged, I wouldn't buy it even if it's fixable. It's not worth the time, effort and money, especially when prices of laptops are coming down every year. A better option is to buy a second-hand laptop that is known to be in good working condition.

3) Replaceable parts. If the problem is with the battery or the hard drive, I could consider buying it at probably USD50-100 depending on the hardware specifications. This is because a new battery can be easily bought and hard drives are usually easily replacement. I would buy it if any other parts are on the verge of breaking down.

At the end of the day, I wouldn't pay anything more than USD400 for any used laptop because most new netbooks nowadays costs around that price already.