This MIT flea market looks like a candy store for techies. I hope that the local kids who are learning and developing electronics know about it (and wish it had existed when I was a kid building circuits in Boston a long time ago).
IMO, things such as this are dying out for a variety of reasons, and there is no single cause. One of the reasons is our own "success" at making electronics so small and dense that you can't do anything else with the parts, assumoing you can even get at them.
I think the golden age of flea markets was in the era of discrete transistor and small-scale ICs (DIP), when you could build circuits on perfboard or similar, and poke, probe, change components, and "mess around".
Now the best thing to look for when at flea markets are power supplies, and also electromechanical components such as connectors, or motors for robotics.
I was at Micro Center yesterday. They have a whole section of parts: memory, P/S, I/O cards, motherboards, etc. Prices are a little high compared to Newegg or Amazon but I wanted a localstore incase of having to return the router I bought yesterday. Micro Center is about a mile from MIT.
I'll go along with Max that there are fewer tinkerers. I go for computer hardware along with odds and ends like sheets of rubber feet.
Computer stores used to be rare and far between and concentrated on complete systems, but nowadays you can go to Best Buy, Office Max, Staples, etc., and pick up hard drives, network cards, RAM, etc. Micro Age and Fry's sell cabinets and motherboards. And NewEgg and Tiger Direct for online orders at good prices.
The weekend sellers at the hamfests could not compete nor keep up on changing stock.