There are a number of sides to this.
The original owner - He should have declared any accident damage to the bike shop on selling the bike to them. Maybe he did and they ignored him.
The bike shop - they are the "pros" here, they should have picked up any accident damage even if the seller did not disclose. They should also have documentation to the effect of the seller disclosing any accident damage or not, this is their recourse with the seller.
The buyer (you) - you did not buy a new bike, there will be wear and tare and scratches etc., but like you stated, you asked and were not informed, so either the bike shop did not know or they were hiding it
The law - CPA is "supposed" to be there for this kind of thing. But give the bike shop reasonable time to sort out the issues first before heading down what is probably going to be a waste of time.