Do you know how much they normally go for 2nd hand?
I'd expect to pay atleast £100 for one second hard, they normally go for around £150 on ebay, the problem is if they have been bonded they are a real pain to remove clean, so you often see people selling the whole boot + spoiler.
You also need to consider the added cost to have the paint removed from a second hand one, postage and how well packed it's going to be, at which point you're probably better off getting one brand new for just over £200 (unless you can find one closer to £100).