Following this thread, It seems like the question of the ethical is being confused with a question of the economic. Which is the dominant logic that should guide the transaction? If it is economic and a one-time event, it seems that offering the lowest price would make the most sense: there will be no future bargaining sessions and thus no opportunity for feedback from this transaction to harm a future round. If it is an ethical one, then regard for the other supervenes economic calculus and should guide one's actions.
To state that there are no universal ethical standards is to fail to grapple with the essence of the ethical. That we can talk of ethics as relevant to this transaction implies a universality. After all, ethics is about sociality and the duty of one to the other. The question then, concerns the substance of one's choices here. What is the ethical standard which applies in this case? Here one can consider the duty to the other and the duty to oneself and the traits with which one chooses to live his life.
Is there a duty to inform the electrician? If so, why? If not, why not?
Perhaps the duty is to embrace community and realize that economic gain is not the sole criterion by which one marks his time in this world. To share the information - this collection may well be more valuable than you think- is to open a space for friendship or at least amicability rather than material competition. It is to act well (in the classical, aesthetic sense) and treat the other with respect as a fellow community member who will benefit unexpectedly from the windfall just as you will benefit from the collection. To fail to disclose is to deny the other's equality in the transaction. There may be many reasons for this, all rationalizations, each more or less compelling [save the elderly gentleman the physical pain of moving the records (empathic), the wasted time of driving to some store for a pittance (economic), the benefit of a windfall that the man could surely use (patronizing)].
As for the buyer. After the purchase, what would the transaction conditions be that give you pause? Failure to disclose? Would you be balancing the joy of saving money against your choice not to disclose, thus mitigating the joy you have in the name of some $ figure? To be ethical is precisely to make a choice in which you meet your duty to the other (this duty can be discerned for yourself if you scrutinize your decision criteria and your moral philosophy, if applicable). What is the vision you have of your life's conduct? If you conceive of your ethical duty as individualist, then offer low. If you conceive of it in other terms then go with that (trust your instinct, there are reasons, too detailed here to elaborate, that we have them).