I've tried to figure this one out myself and 
haven't come up with a good answer yet.  I've had 
one bad experience with a cel not arriving and 
the seller being non-communicative, but other 
than that, the vast majority of my Ebay exchanges 
have been down right wonderful.  Indeed, I found 
this very website via someone I met on Ebay.

I suspect (and no intention meant to anyone here) 
that the problem may lie in the basic democracy 
of Ebay.  Some cels sell for less than the seller 
would wish, some cels sell for more than a buyer 
would wish.  Someone's feeling sometimes get hurt.
Is this evil?  Nope.  Will someone *think* it is 
evil?  Probably.

Personally, I have no issue buying on Ebay.  I've 
found some great bargins.  I've also found some 
wonderful cels I haven't seen elsewhere.  I've 
*also* been offered (and bought) great cels from 
collectors I have previously bought from on 
Ebay.  Just pay attention to what you're 
bidding.  Do some research.  (I had one cel I got 
on Ebay that I was concerned was a fan-cel due to 
the high quality of the cel and the cheap price.  
I have since confirmed that it's a production 
cel).  And meet some wondeful people and fellow 
collectors.

Buyer Beware.  But also, Buyer, Have Fun!