Added Sealants, mastic, caulking, bitumen plastic, indicates the job was not preformed properly.
Valleys do not require visible sealants. The chimney headwall flashing and counter flashing should be visible.
In your image, poor shingles chimney clearance.
Let me put it to you this way. On a properly installed shingle roof you do not see sealants. The underside of the shingles is used to apply sealants to. Everything is hidden.
Sealants are used by roofers as a means to tract down a suspect leak as in the cricket.
Look at the sellers declaration or ask the agent was there ever a leak at the chimney.