> It isn't slander to call them what they look like: used car dealers.

Well now if they represented themselves as NEW car dealers that would
be something to slander them about... but I'm pretty sure they ARE a
used car dealer.  Further, it is slander to suggest they are
disreputable and taking advantage of their actual customers without
any evidence of them doing so, or in this case taking what they
actually said "we have a buyer" and deliberately misinterpreting it
"we're pretending to have a buyer to get your van for ourselves".