Buying a second-hand one as long as it’s in good condition is okay. It’s not a very important consideration, like a lifejacket etc., in which case a used one may not do the job well. If the horn works well, you can go in for it. However, make sure you get a good bargain on the price. With any accessories, especially the electronic ones, their worth goes down to almost half (or less) once they’re out of their packaging. A decent tugboat horn won’t be challenging to find, and many second-hand stores can also get you one.