My thoughts:
I'm sorry you didn't get the cel you wanted. :/
Not knowing the other person's side of the story,
I won't speculate on why a cel might be withdrawn
from sale, then sold 4 months later to someone
else. 

In regards to your question of whether or not you
should demand another collector sell a trade-only
cel, imho the answer would be no. For all that
he/she might sell it a month or 3 years from now,
right now it's trade only, implying that he/she
has an attachment to it now. You could *request*
it be sold to you, but most cel gallerys I've seen
say pretty clearly that offers on NFS/trade-onlys
might be considered, but not to be offended if you
get turned down.