I would say sell if high $xx or more, but it totally depends on what you paid, your financial situation and original intentions for the domain, how long you are happy to sit on domains etc... That's just my personal opinion of current market value.
I would hold out. I honestly find that the first offers are never the best ones. If you don't find an offer any better, contact the original bidder and chances are he will still be interested.
The only reason I would say hold out is because it does look like that face that many people use in Instant Messenger conversations. (o-0) or something like that. Therefore, it may have some appeal to people who specialize in that genre.