Your R9 280 is about on par with a GTX960 which, new, goes for around $150 US. Given it's a couple of years old, I'd say you could probably get between $75 and $100 for the card.
If you want to make this a "for sale" thread, you need to update it with the appropriate images as outlined in the rules, stickyed at the top of these forums.
Wolfshadw
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