My personal reasons are that Ebay keeps changing 
their rules and regulations and threaten to close 
your auctions because you missed one of their 
obscure new point.

They have a big monopoly and an even bigger fear 
of any kind of competition. They started by 
closing auctions of people who had a link to 
another auction site on their own website. (not 
even on Ebay page). And now, sellers are not even 
allowed to put a link to their site in their 
auctions. If they can pull it off, they probably 
would like to forbide you to mention that your 
name or that you have a shop. (if they could 
prevent you from having a shop so that you sell 
everything on ebay, it would be even better)

Lastly, in contrast with their eagerness to close 
auctions that threaten (or so they seem to think) 
their business, they don't give a damn about all 
the illegal auctions, whatever forgery, unlicensed 
produces and others. They only close such auction 
when threatened by a lawsuite.

Both reasons mentioned about are also probably why 
bidders/sellers are totaly forbiden to discuss of 
an on-going auction even privately.
To prevent abuse, you would need something like on 
Yahoo, where other users can ask a question about 
an auction which can be read by any other users.
That's the only way to inform people of potential 
problems with an auction. (and yes, there is many 
fake cels on Ebay). But that's what Ebay will 
never allow as it may make them gain a little less 
money from dubious auctions.

Those problems wouldn't bother me on a free 
website. But on a commercial site where you have 
to pay an important fee, I can't _personaly_ 
accept them. That's why, some time ago, I asked 
them to definitively close my account.

Cheers,
Yann Stettler