I don't know if people here can provide any 
better advice than to buy it if you feel that the 
cel is worth the asking price, or wait for a 
better deal that usually comes along later on. As 
you said, "cels are not a steady market," so it 
would be hard for anyone to assure you that you 
will be able to recoup what you paid for it later 
on should something happen to you. $500 for a 
Tenshi ni Narumon OP cel may not be worth it 
today, but maybe it will be worth it three years 
from now (look at what happened to the Tenchi 
Muyo and DBZ market, for example). Then again, 
maybe it will still be overpriced three years 
from now.

---Trebuchet