If the price on the cel was as good as you said 
it was, I couldn't imagine a collector passing it 
up. 
Plus, that would mean that the dealers from 
Nichibei would have not only been at their table, 
but also running passed tables trying to buy up 
Gundam cels for cheap. Does that happen? I've 
never watched the table around a cel dealer. As 
far as I know, sellers at conventions are too 
busy trying to get rid of things to go looking 
around themselves. 
It's also possible that Hobby Shop K had a cel 
from the same sequence and are really good 
friends to the people at Nichibei and sold them 
one. (I have a cel in my collection which seems 
to be on every dealer's website.) 
Or that after the convention the cel remained 
unsold and Nichibei expressed interest in it. 

Who really knows? 

I have to admit though that I don't like 
speculators, but at the sametime I think people 
should get the best price for their cels or 
anything they collect. It's a grey area at times. 

I am definetly against the idea of collectors 
exploiting each other, creating a vicious circle, 
where they buy cels, increase the price two or 
three times to sell to their fellow collectors, 
so that they can get money to support their 
habit. Collecting should be something that brings 
people joy, not a cutthroat game.