Ah, but as a collector, if you're trying to sell 
your items back to dealers -- at least with 
action figures and comic books -- they'll give 
you at most 40%-50% of the guide list price for 
them. More if it's rare, less if it's common. 
Anyway, as for reselling, the profit is limited 
by the fact that you can't ask for obscene mark-
ups. There are too many dealers and price guides 
out there for anyone to believe that a 1000% mark-
up is reasonable. (As for cels, the best rule of 
thumb is to look at a number of different 
websites -- compare prices. Get a feel for the 
market before you jump into anything.)   

As for reselling collectibles -- it's too easy to 
start another Tulip craze -- where everyone is 
buying only to sell. Prices are artifically high 
because of speculators. It's sort of like a 
pyramid scheme -- everyone can win, only if 
there's an infinite number of people entering the 
system. The last people in the scheme, end up the 
losers because no one's willing to buy off of 
them. Those senarios eventually collapse, with 
people fleeing the market. As I mentioned before, 
it happened with comic books and sport cards. 
People aren't fools forever, you know. (Cels are 
rarer than either of these two situations, so I 
doubt the market will crash and burn like those 
did).    

Personally, I'd rather deal with people who are 
in the thing for the long run, rather than people 
who're in it for the short-term. The people in 
the second group reek too much of speculators.