,
It is not against the rules for a user to make an offer on an item even though you may deem it to be a "lowball" offer, however making multiple posts linking to your original sales thread does constitute as spam so please refrain from doing that.
It will do you no harm to just ignore his "offer" and wait for an offer you're satisfied with, you do not need to open more posts to link to your original topic.
Of course as Sqnman007 said if you believe that he's maliciously spamming your posts then by all means do hit the report button and report his replies, moderators will carefully review your reports and take appropriate action against him if needed.
Feel free to send me a PM if you have any questions in-regards.