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.
Hamfests AKA Electronics Flea Markets seem to be dying out
For one thing there are fewer hobbiests building their own stuff. Having said that, there's a humongous Hamfest here in Huntsville in the summer -- I went for the firest time last year and I was blown away by the size of the thing -- also I picked up lots of cool antique analog meters -- I'll be going back again this summer for sure!!!
a lot of my chips, ECL's and some CMOS has gold bonding wire, and 80 20 gold / tin, paste, and the gold plated circuit boards, are now profitable to refine. So my junk pile is a gold mine. but some of you all, knew that . Cheers hunters of good electronic scrap. never enough 1 percent resistors, or 50 MFD's at 16 V. in hand.