An HD7850 1GB would probably be better at around £130 TBF. However,the XFX graphics card warranties are not that great and are quite poor,and the single fan HD6870 1GB is not that great(the dual cooler ones are somewhat better).
Now,if the seller was kind enough to handle the rest of the warranty fair enough,but it is not always the case. Regarding used bits,I use a few too,but I also know if anything goes wrong,I will need to foot the bill if there is no warranty.
I can't recommend used bits with no warranty, especially to a person who might not have built many PCs,because if the parts go kaput within a few months,it will be my fault TBH.