Can I be sued for slander after warning a business's customers about my bad experience with it?

Slander is the "speaking" form of what is generally known as "defamation."  It is where words are used in false and malicious way to harm one's reputation.  The key to a defamation suit is that the words are false.  Truth is always a defense to a defamation suit.  If you told the truth (and it appears that you are backed up by other buyers here) then it is doubtful that such a suit will be proven by the seller.  I would suggest, though, that you stop your campaign.  You have made your point and the contacting of the other buyers sounds like you are targeting this seller.  It may not be rising to the level of something wrong that is actionable but one never knows.  You made your point.  Write back to the seller and let him know that all you did was disseminate public information to those in a similar predicament as he and that truth is your defense here to any threatened suit.  State that if he intends on pursuing the matter you will counter sue for legal fees once his case goes down the tubes.  Oh, and get the names, numbers and addresses of those other buyers. Good luck.