Alot of times when people bring some kind of note-worthy retro bike into the shop for service, I talk to them about it being a classic, etc... it goes one of two ways...
Either they say "Oh...I just like riding it. It used to be my dad's and I need to get around campus." on a Ritchey or classic Klein, or whatever that is way past its prime visually and mechanically
or
They say "Oh, really? what do you think it's worth?" and their Ebay antennae are immediately dispatched...
So now, if it's something I'm interested in, I subtly tell them that the newer bikes have come a LONG way in terms of comfort and light weigh and would really be worth a look. "Oh, and we could offer you a wee bit of a trade-in for that old steel thing you have back there. Salsa was sold a few years ago, and the new owners won't honour your warranty if anything ever happen to that bike...and parts for the fork aren't available anymore"
I don't do it often, but it's worked for a coule bikes. We sell a new one, they get a "fair" trade-in value for their "old clunker" and I get a new project...much to my wife's chagrin...
rb