When you purchase a trademark, there is no requirement that you record the sale, but it sure is a good idea. Why? Take this example:
Owner A sells a trademark (and the underlying goodwill) to buyer B. Buyer B never records the sale. Owner A, feeling mischievous, then sells the mark again to buyer C. If buyer C truly didn’t know about the sale to B, then C gets the mark and B loses out. Had B recorded the sale, he wouldn’t be out of luck, and C wouldn’t have gotten the trademark.
As an aside, the sale of a trademark must be accompanied by the underlying goodwill – the bare sale of a trademark is ineffective.