Why would you want to know that the price has gone 
up since the buyer bought it? All that really 
matters is what the asking price is, and whether 
that seems like a price you'd care to pay. I think 
it's safe to assume almost all sellers raise the 
price at least a bit. Certainly you can't tell me 
that you'd pass up a cel you really want because 
it cost more than the seller paid for it? 
For example: I bought a GORGEOUS cel of Tenchi and 
Ryoko from a forum friend a while back. I paid 
quite a bit for it (in the hundreds of $s)but I 
was perfectly happy to. Her price was within the 
range of what I felt the cel was worth to me. I 
really don't care if she only paid $25 for it, and 
made a couple hundred dollars off of me, it is 
perfectly within her rights, as it was in my 
rights to pass if it felt too high. 
Wanting disclosure of previous price is nuts- the 
cel is worth what a person is willing to pay for 
it.
I really think this all boils back to that thing 
most people here don't like to discuss, the fact 
that cel prices are rising, and long-timers want 
to preserve the prices of yesterday. Get used to 
it rising. Anime is more and more mainstream in 
America by the minute, and more and more people 
are wanting to own a piece of their favorite 
shows. Compound that with the fact that more and 
more shows seem to be going CGI, and you have a 
recipie for cel prices to keep rising. Hey, it 
means I can't buy as many cels for my dollar as I 
used to also, the difference is I don't try to 
deny it, I face the truth and live with it. 
Blair