Buying Someone's Records: An Ethics Question

While doing some work at my house recently, an electrician noticed my audio equipment and vinyl collection. This gentleman mentioned that he has a substantial collection of records sitting in boxes in his attic and asked if I would be interested in making him an offer to buy his collection since he no longer has any use for it. He is in his seventies, and the collection is one he has accumulated over his lifetime -- althoughhe probably hasn't purchased anything in 20 years. He also mentioned that the collection includes a number of very good condition 78s from various blues artists dating back to the 1950s. I am quite sure that he has no idea what the "market" value of his collection might be.

If i offered him something like $5 per on average, he would probably think that was a great price. In truth, many of his records -- particularly the old blues 78s -- have a much greater market value. Of course, I know that but he does not.

Your thoughts on the ethical approach to making an offer for his collection would be appreciated. I don't want to take advantage of him, but if he remains blissfully ignorant of the real value of his collection and is happy with a "low ball" offer, who is hurt?