I think there's two different issues that are 
being perhaps mistaken for the same:

First are the people who will buy cels from sites 
or jy, then immediately sell them on ebay less 
then a few weeks afterwards, in order to make 
money off them.

Second are those who buys sets of cels off yj, 
keeps a few, then decide to sell the rest. I see 
nothing at all wrong with this, but there are 
other ways to sell cels other than ebay. Like 
perhaps putting them on a small site with fair 
prices, and announcing on the cels mailing list 
or here that you have a few extra cels for sale.

I know ebay is a nice place to sell things if you 
are in a pinch, but if you're just looking for a 
way to rip people off because of the insane 
amounts that cels often cel for there, then I 
feel it's unethical.

just a few of my thoughts, feel free to add or 
correct me on something.

Ryouko