Crossfire 7870 vs r9 280 and buying used cards

should i buy a r9 280(about 200€) or crossfire 2 used 7870(about 200€) i would buy one 7870 now and another one laterthe used card has not been used for mining  

You'll have less headaches with a single r9 280 card setup, especially the alternative being 7870's. Pretty sure there are a couple of peeps here on the tek forum that have issues with their 7870 xfire setup.

I'd go with the single more powerful GPU. 

While theoretically two 7870s would be faster it isn't that simple. Some games will see good improvements, around 70%, increase over one card but most will be much lower than that and some will even have lower performance and still others won't work at all. Plus the issue of microstuttering, increased power draw, increased noise and heat. 

I XFired 7870s (270X) a while ago but it was a massive exercise in misery and failure. I had so many problems with the cards that I just sold them and bought a 780. One or two people have had good luck with the 7870s, but for most they seem to be incredibly temperamental.