I suspected that litigation was the concern. Has the site's admin. sought council on this issue? It seems to me that the site probably can't be held legally responsible for what members post independently, though I haven't researched this.
It would obviously be a different story if the site itself or an operator such as yourself slandered a dealer. To circumvent this problem the admin. could just "not notice" this sort of posting for a short period of time and then pull them that way they've done all the could. Anyhow it may be worth looking into the legal issue instead of being scared off by angry vendors because having a warning list would benefit many would be victims.
I feel a little light headed... maybe you should drive...