Substantively, the purchaser fully agreed to all the seller's terms, as is evidenced by his signature on each page of a written contract. I am sure he also signed in full on the final signature page where he warrants that he has read and agreed to all the terms and conditions in the whole written agreement.
That the sales assistant, whether acting as the seller's agent or courier, erroneously or wrongly endorsed certain clauses but without modifying them, cannot alone be taken as evidence of deception. Any court, after first considering the agreement in toto and finding substantive causa and a meeting of minds, would then look whether any specific endorsement or initialling at specified places was an essential term of agreement as written. And even if the written agreement is technically defective or vitiated in the manner averred by the purchaser/OP AND that clause is an essential term of the contract, there are various remedies at law for both parties. An essential terms means a term that, were it not present or ignored in performance, would have prevented the parties from being ad idem - not possible in this case.
The sad reality is that it's probably just not worth the seller's while to drag this out and tie up resources. We'll all end up paying a little bit more to cover the costs of another contract breach.