You can be a good little capitalist and feel 
perfectly justified by selling at a profit.  I 
must warn you though, what follows is what I'm 
covering in my econ class so might be extremely 
obvious, let alone boring. And I'm notoriously 
not succinct, if you haven't figured out by now ;)

Here's the situation:

You have a cel, it's value to you is say $50, 
that may be what you paid, but it doesn't really 
matter. (Why would you sell a cel for less than 
you value it, even if you did pay less? You're 
just hurting yourself there.)

J Random websurfer has $250, and when he looks at 
your cel he thinks "That's worth $200 to me."

So, if you sell your cel at some price, lets say 
$125, things work out this way:

You: Start at $50 (cel) 
            - $50 (sold cel) 
            +$125 (cash)
            ====== 
            Total $125 worth of "happiness"
            Net +$75 happier

J. Starts at $250 (cash)
            -$125 (cash)
            +$200 (cel)
            =====
            Total $325 worth of "Happiness"
            Net +$75 happier

Therefore, by selling the cel, you increase the 
happiness between the two of you by $150. 
Actually, it doesn't matter how much you sell it, 
there is going to be a net gain in happiness. 

Now, what reasons would you have to increase the 
buyeres happiness more than yours?  If the buyer 
is your friend, you may gain happiness through 
him getting happier, so it makes perfect sense to 
sell it to him for less than he values it.

On the other hand, what incentive do you have to 
increase a stranger's happiness more than you 
increase your own?  The buyer comes out happier 
if he buys the cel for anything less than he 
values it: he gets a deal.

And, if for some reason, the fool decides to pay 
more for the cel than he values it?  As long as 
you were honest in your description, it's his own 
dern fault for not being able to determine his 
preference orders.

Finally, please don't get hung up on using 
dollars as a measure of happiness, they are  
simply the lowest common denominator in this 
example, and can easily be converted into 
whatever else pleases you.

My apologies for the length, I'll throw up one of 
my "increases in happiness" in repentance. Here's 
Rio from Burn Up, making the best out of a bad 
situation.  It's the seller's image; I need a 
scanner, but for the price of one I can get a 
really nice cel...