Why would he be held accountable, the buyer should be held accountable for making the decision. Why would you buy hassle in the first place? Demand from the seller that you need engine from which the original wiring is not detached. Examine the engine properly, check the wiring, ask gaurantee, count things like sensors, look for vacuum pipes, examine the condition of block, see for dents, see for missed accessories, and then purchase the engine. You cannnot just simply blame the seller. Its always the buyer in my humble opinion.
And you are paying for his honesty now, what you say about that ?