PErsonally I would agree with getochkn. If this were a friend selling you this card that is one thing, but that is a really good price, and as far as I'm concerned there are a few bumps against it:
1.) Reference card (probably inferior cooling/clocks to other cards, not always true, but usually a safe bet)
2.) Unknown seller
3.) AMD card
Number three is not a knock at AMD cards, it is a knock at mining. Right now coin mining is HUGE and AMD cards tend to be used for it more then GTX (personally I don't mine, so I don't know why AMD cards are preferred, i'd assume price but I'm sure someone here can tell you).
Just know that mining is incredibly hard on a card, especially with someone that does it exclusively. If it was used for mining, this card could have been running for 30 straight days on max load, in less then fantastic cooling (as far as I know a lot of miners like to keep there rigs cool enough to run, not as cold temps for longevity.)
TLTR: I am strongly in favor of new components, especially with graphics cards. The 280x is a great card, and msi is a great company. It will be able to max a lot of games out (probably most) and will last a long time. Having the new card would be worth it in my opinion.
If you choose the reference cards. Run it under full load for a bit to stress test it, and try cranking your fans, make sure they are spinning correctly, no funny sounds or wobbling. Those would be my concerns.
Good Luck!