And that's what my friend discovered. From dealers, he wasn't offered better than 40 cents on the dollar.
I'm not a professional. I just collect for fun. For me to get a collection like his via dealers or even Ebay, I'd be paying multiples of what I paid. How many? I don't know, but I just found a plate block of twelve 8¢ 1972 stamps selling for $1.48 + $1.00 shipping, so $2.48 for 96¢ worth of stamps. I paid 67¢ for that same plate block.
Ultimately, all that matters is that my friend was happy with my offer and so was I. That's how it should be.
It's got to be discouraging, though, for all those collectors who built up their collections a little at a time over decades.
My old collection, by the way, was comprised almost exclusively of *used* stamps, obtained at little or no cost. I soaked 'em to get them off the paper... sorted and replaced them as better copies became available, etc. My investment was one of time, but I enjoyed it so I don't really think of it as a "cost." That's how stamp collection ought to be, imo. I really don't understand people like my friend who spent all that time buying plate blocks from the post office. Where's the fun in that?