May I ask what sort of markets are you referring to? Ebay, known brands like Dell etc or elsewhere?
I think I'd be more sceptical if considering ebay but only because I don't know how reliable they might be. Others, such as Dell for instance, I would have no concerns as their Customer Service is reputedly (and proven) very good.
I got a 'refurbished' desktop from Dell recently, on behalf of a (non-techie) friend. Very good price/spec, Win 8 and appeared to be 'new' although possibly not and it's seems really good. It did have a strange problem in relation to two different monitors I connected with but Dell sent chap out next day, who changed motherboard, which fixed the issue. So, not complaining.