Under California Penal Code section 496, the provision which makes receiving stolen property a crime, you must have knowledge that the goods were stolen. Mere possession of stolen goods is not enough.
Knowledge, however, can be inferred from the circumstances. For example, in People v. Myles (1975) 50 Cal.App.3d 423, the court of appeals held that possession of stolen property, accompanied by no explanation, or an unsatisfactory explanation of the possession, or by suspicious circumstances, will justify an inference that goods were received with knowledge that they had been stolen. So even if the person who sold or gave you the bill of sale did not explicitly tell you they were stolen, if a reasonable person would have assumed so in the circumstances, a court will likely hold that you should as well.