My seatbelts came with my Class C and are bolted to the floor. It's a DOT (Department of Transportation) law.
If a Class C is designed to sleep 4, then when they build it, they MUST include 4 seats and seat belts, ditto if it is built to sleep six, then they must have seats and seat belts for six. This is a requirement by law. The seats can include sofas, and dinettes, but they still MUST include the seat belts.
What I tried to explain earlier, is that often the seat belts are designed to be stowed away under the seating, when not in use, so they may not be readily apparent in pictures of the Class C interior.
If you are looking at a Class C without seatbelts, then it's not street legal. The seller needs to add the required seatbelts.