The recent sale aside - the people who are vintage high-grade comic books collectors are getting older and dying off so the outlook is not that good.
Which leads us to wonder what today's kids will want to buy in 30 years as a reminder of their youth.
- Music now comes completely free of any tangible item--no album covers, no CD cases, etc.
- Same with other popular media. While movies are still available on DVD, in 30 years I'm not sure what you'd play them on.
- Cars are still relatively unchanged, and kids will want to relive the glory days of their 30 MPG 4cyl tuner cars, but holding onto a car for a few decades in hopes it will appreciate is an expensive proposition.
The best bet would be something that is relatively inexpensive today, that people feel an emotional attachment to (or associate with a time in their lives), that requires little/no maintenance, which people don't already collect in large numbers and which can't be reproduced easily at a later date.
Maybe today's laptop machines, software, printers, operating systems and applications will seem quaint enough to be in demand in a few decades. Already most folks under 30 have probably never seen a "C:\" prompt. An 8" or 5 1/4" "real" floppy disk is already an interesting novelty.