It's going to sound mean, but by take on it would 
be as follows:

1. I only agreeed to sell because I needed money. 
The buyer didn't send what was promised.

2. The buyer has ignored several e-mails and also 
their own deadlines for paying me.

3. Conclusion: The buyer has *broken* their half 
of the agreement with me.

Given that, I would send an e-mail to them that 
makes it clear the deal is 100% off -- completely 
over. They do not get the cel, and only get a 
*partial* refund, *IF THAT*. (I would be most 
inclined to simply tell them that they blew me 
off and there will be no refund either.)

I don't know how much money you're talking about, 
but this person isn't interested in anything but 
dragging things out. They can't be interested in 
the cel. They seem not to care about the money. 
Just end it. Payment was insufficient for too 
long a time, that's his/her problem, not yours.

I think your offer of a refund based on what it 
sells for on Ebay is far too generous. But then, 
depending on the situation, I take extremely 
unkindly to people who do things like that to me. 
From your posts, I think you're more forgiving 
than I am in this area. ^_~~

Many Sharp Smiles,
--Drac