Can they sue me for slander?

We went to a local dealership to trade our boat in and they "found a problem" while we were signing papers. Told us that the exaust manifolds were cracked and we had a cracked engine head and water was getting into the oil. Well, we had just changed the oil the weekend before, it was new clean oil, couldn't have been "milky" like they said. Also, we take way too awesome of care of our stuff... It just wasn't adding up. They told us that they would take it as is but would have to take $6500 off the trade allowance. My husband and I talked about it and decided to just take it somewhere else and have them look at it. When I went to pick the boat up, they told me they were 75% sure that it was a cracked block and because we were such good customers they would just eat that $6500 if we paid them $1500 to replace the manifolds. That seemed kind of weird to me. We opted to NOT do any of that. When I was leaving he handed me a service sheet and on it, was the compression test. Perfect. No leakage. How on earth do we have a cracked block if the compression is perfect? Makes no sense. We take it to another dealership and they find nothing wrong with it. I then posted on Craigslist calling them criminals and explained our experience with them and stated that I would NEVER darken the doors of that place again. Never said for anyone else not to go there. Oh and I called them lying b@stards. Nothing was false. All true. I get a certified letter from them today telling me to remove the post (it was obvious is was me even though i didn't leave a name) and they wanted an apology. Said that they were a 5 star business and if I didn't remove the post they would sue me for slander (even though its liable, right?) and have their attorneys come after me. My personal experience is my freedom of speech right? Doesn't it have to be false information in order to file a lawsuit?