I would search the sold listings on eBay to get an idea of the price you should sell it for. Here's that link: http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=macbook air mid 2012 1.8ghz&LH_Complete=1&LH_Sold=1&rt=nc&_trksid=p2045573.m1684
If you are afraid of selling on eBay/Craigslist, you could sell it for around $100 less on one of these buyback sites:
Buy & Sell Used Electronics - http://www.gazelle.com
Sell Your Used Electronics - http://www.nextworth.com
Sell Your iPhone, iPad, MacBook - http://www.itemcycle.com
Buy & Sell Electronics Marketplace - http://www.usell.com
I'd look around those sites and see who can give you the most money for your MacBook. Best of luck!