I'm assuming the cel was in the higher-price 
category. It could be possible that your buyer 
ran into some sort of financial bind... 
especially when you consider the many people 
being laid off this year. Even if a payment plan 
wasn't mentioned, I think the buyer sending 
chuncks of payment shows that they didn't want to 
abandon the cel and simply sent what they could 
afford at the time. IMO, the fact that they 
haven't called off the deal and asked for a 
refund shows that they may still be interested. 

Its weird that they would sent partially payment 
and they drop out of sight. 

I think there's more to this than the buyer just 
losing interest. Keep us informed on what happens!