Bored Borg wrote:
if you're craiglisting, why not add a line to the effect that any cheques
requiring "change" will only be accepted on the strict understanding that
this will entail an eight week clearing time before goods are despatched or
monies go out?

A better position might be to state that all payments via check are for
the precise amount and any overage will be kept and/or donated to
charity as it was not requested to begin with. Regardless, the idea of
waiting 6-8 weeks for the checks to clear should be standard.