10% ebay fees; materials, postage and insurance on the shipping; the chance that it will break in shipping and you'll eat the cost; the chance your buyer will attempt to return within eBay/PayPal's 60-day window; the chance your buyer totally scams you outright.
With eBay you run the risk of the buyer saying the computer was defective or not as described and sending you back a brick or their old computer and you're stuck holding the bag. eBay has shifted away from a good place to sell your spare stuff and heavily leaning towards the larger sellers who can offset the bad apples on margin volumes much like how Best Buy can offset their return fraudsters.
As mentioned earlier, Craigslist would be the absolute best bet as you can get cash and not deal with eBay fees or return fraud risk. The problem there is you end up having to counter balance the risk that you'll run into someone trying to rob you for the MacBook. Not that I have had any problems buying on Craigslist, but when it is a high value item I am always very visibly armed and showing up with more than just myself to make sure I can keep a solid eye on the situation. If possible, I meet at a police station, but keep in mind the police will keep you from getting robbed which is good. They won't settle disputes though. Like if the person claims they gave you money they didn't give you for they give you fake bills and you only notice later.
If you're into selling though they do offer a price arbitrage opportunity. The same is true for all sales when they dip below the price you can sell at in the secondary market.