Used PSUs are never that good an idea, how do you know where they have come from? How have they used it? Does warranty carry over to the new owner? Personally, used PSUs are a no go for me.
Having said that, the RMx is a mighty fine PSU so assuming they haven't been using it for the last 10 years or using it as a volleyball then it should theoretically be fine.
But looking at your previous post, what specific Xilence model is it? You didn't specify, they have some decent ones. Also you're probably better off putting the money into a new but lower end PSU like a Corsair CX where you can guarantee keep all the warranty too.