Post production cels were made STRICLY for resale.
They have no other purpose. Production cels and
gengas, on the other hand, had an initial purpose
which was to produce the show. Their original
intentions were not to turn around and resale it.
So, typically, with items like these, they "may"
gain a bit of value, but in the long run, they
will most likely lose in value.
I am basing my assumptions on the Barbie doll.
The early Barbie dolls (the 1960's) were
originally made as toys. Their intentions were
not based on collectible value. Today, you see
many Collector's Edition Barbie dolls and after
collecting for so long, I have a couple hundreds
"Collectors" in my collection with 80% of them not
worth (or worth as much as) the rpice I paid for
them a few years ago. The 1960's ones, however,
appreciated a lot in value with the years.