There are sword collectors out there, and there are especially more collectors for movie replicas like the Eragon sword. If the sword is not damaged, you still have the box, and certificate of authenticity, you should be able to re-sell it for a good price via Ebay or similar auction sites. Because the sword is still available for purchase from regular dealers though, you are unlikely to get more than a little less that what it is being sold for as "new" from a dealer.
Looking at the prices going on Ebay right now, you could probably expect to get around $50~218 depending on what sword you have.
To be frank, it's unfortunate, but if you paid $300, you have likely paid far too much for the sword. There are some swords worth that much (and more), but they generally are not movie replicas.
If it is some sort of special limited number collectors edition (meaning it has a number x out of x on the authenticity certificate), you may be able to get a higher sale price.
Search around for your particular version, and see what it is going for on average to get your starting sale price.
- Sword Collector (20+ years)
- Ebay Going Prices: http://collectibles.shop.ebay.com/i.html?_nkw=eragon+sword&_sacat=1401&_odkw=eragon+sword&_osacat=1401&bkBtn=&_trksid=p3286.m270.l1313
- Standard "New" prices right now: