Before even THINKING of threatening ANY sort of 
legal action (which you obviously don't even 
begin to understand since libel is for written 
actions and slander is generally for spoken) I 
think that you should look very carefully at the 
two posts between you and Yann.

Yann's post was entitled "Please stop" and was a 
polite, short request to stop using the board as 
a source of free advertising.  Here is his first 
message in its entirety:

"As some one already told you, stop spamming the
forum for ads for your site. People understood
that you have an auction site.
Instead, you should register it on Cels.org or
join the webring, assuming that you really have
cels on auction.

Cheers,
Yann Stettler"

He did not question your "integrity", such as it 
may be, and he did not provoke you in any way 
that could be supported by the "reasonable man" 
standard set forth by the majority of the US 
courts for Tort actions of this kind.  He made a 
polite request and you responded with a 
profanity laden message entitled "hell no". If 
anything, you have just made several very 
serious and unsubstantiated accusations against 
Yann.  

I would like to particularly address one 
sentence of your last message:

"If you cannot speak to anyone the way you 
normally would, you should not be speaking."

I would suggest that if that is the way that you 
normally speak, then you should certainly 
refrain from speaking in the future.  Several 
people on this board, myself included, have 
noted our disapproval of your actions and told 
you we are not interested in your auction site.  

Those who were on the fence have certainly not 
been swayed by your confrontational manner and 
unprofessional attitude.  We have a free place 
to auction off our cels, and would not trust 
them to you if we didn't.  Please quit while you 
are ahead.