A different reason I can come up with involves 
trust. People are more willing to buy a high-
priced cel from a well-known dealer, for example 
Animanga, Anime Chaos, or Anime Taro, than from 
an individual on eBay because they are less 
likely to be swindled. The high-profile dealers 
do not want their reputation ruined. If you plan 
to bid on an expensive cel on eBay, you would 
check and double-check the seller's feedback to 
make sure he is legitimate. Furthermore, some 
buyers on eBay do not know the cel collectors 
community too well, so while we may know that 
people here can be trusted to come through on the 
transaction, they do not.

---Trebuchet