While I personally like selling parts for old Schwinns, etc, to be fair, some old bike customers can be extremely time consuming for the amount they spend.
A typical parts request from one of your much maligned "spandex" crowd: "Do you have a left 105 STI lever?"
A typical parts request from a "customizer": "I have a 1952 Schwinn I just bought on Craigslist you can tell it's a Schwinn because it has an S on the seat I bet you've never seen one of those it's the only year it came in this color you can't get tires for it anymore I'm gonna put an electric motor from a vacuum cleaner on it or maybe a lawn mower engine maybe I'll make it a three wheeler or a lowrider it's gonna need to be rewelded do you guys do welding here but I'm gonna repaint it anyhow jeez 400 dollars for a bicycle you can buy a car for that do you have mag wheels in this size how much do you guys charge to build wheels that's a lot do you have any old bikes in the back I can look through I don't want to spend a lot of money six dollars for a tube that seems like an awful lot I'll just go to Walmart do you have any used ones just laying around is that all the pedals you got do you sell just the patches without the glue?" Oh, and can I have a valve cap?