I keep files of all of my cel stats. What I paid, 
shipping and insurance, sequence numbers, whether 
there are backgrounds or sketches and whether they 
are stuck or not, any extras, episode and/or scene 
notes, wear and tear, whatever is there to be 
noted.

In addition, I keep copies of every bid 
notification, every email exchange with sellers 
(both to and from me), every invoice. Anything 
that has anything to do with a purchase.

However, I don't base my purchase limits on what 
is noted.  I base them on whether or not I am 
having trouble with my finances at any given 
moment.  If I am, I slow down.  If I'm not, I keep 
buying.  If I am hurting a bit but can't slow down 
(a cel I must have, or whatever), I find a way to 
work around it.  Sometimes that's a struggle, but 
I would probably be bored if it was any other way. 
;)