Probably eBay, but I doubt it will have much value. Modern computers don't have much of a collectors value even if they are in new condition, and as a computer it is almost worthless because of the age and the fact that the battery is shot from sitting that long. Maybe it would have some value in 50+ years, but I don't think it would have much value now.
I have a feeling that the first gen MBA might be too new to have any value among collectors. eBay and Craigslist are probably your best bets, but personally I would just keep it because you really won't get more than a couple of hundreds off of it.