I forgot to mention the best reason why I think 
taking screenshots and writing down what you'd 
pay for those cels is a good idea.

I'm sure plenty of the insanity in the pursuit of 
cels is caused by the feeling, "Oh my god!  A cel 
from that sequence/show I like!  I have to buy it 
now, or someone else will order it before 
I do!  Hmm, how much is it... it's more than I 
want to pay but... someone else will get it 
before I do!  I'd better just buy it and worry 
about the consequences later!"

...and so you end up with a cel that you paid too 
much for, that may not even have been on 
your "cels I want" list; purchased out of that 
update-rush impulse.

If you're careful, you can avoid that dangerous 
state of mind by knowing exactly what you 
want, and how much you're willing to pay for it.  
And stick closely to your numbers: would you 
rather trust something you wrote down when you 
were giving the matter serious thought, or a last 
minute panic-driven decision?  ~_^;