We're all collectors here.  I think most of us 
have had to sell something off to buy something 
else at some time.  Therefore I think most 
potential buyers are willing to wait to hear 
back, especially if it's a choice between waiting 
or not getting the cel at all.  However speaking 
from personal experience I'm sure they'd 
appreciate you sending them an email during the 
wait time and explaining what you're waiting on, 
rather than not replying at all.

As far as how to let go.. that's not so easy.  
There are still one or two cels I've sold at 
various times that I sometimes miss, but I love 
the cels I have now too, and know I wouldn't have 
been able to afford them if I hadn't sold off the 
ones I did.  As long as you're sure you'd rather 
have the one you're buying instead of the one 
you're selling, go for it.