It's all generally great, but as pinkamy said be very wary about electrical stuff. Otherwise, go for it.

There are some advantages, such as the secondhand army boots I'm wearing having been broken in by someone else's poor feet, but there will be the occasional duff item, especially if you buy through the internet. But look at it this way: the money you save in buying good stuff second hand will more than make up for the odd bit of bad stuff you get by mistake.