I've had a bike stolen where the frame manufacturer offers a simple online service to report a bike as stolen. There is zero excuse for anyone buying it to not check the number that is engraved onto the frame, not to mention with police who similarly have the serial number in a database.

Imagine if people had to drive around in creaking beater cars with leaking roofs because you couldn't store anything nicer anywhere but a garage with surveillance, and even then if someone finds out you have a nice car in a garage, there will be a breakin. That's pretty much the situation with bikes, and police simply shrug at what in most cities is very much organized crime and a huge quality of life issue.

> but people willing to buy such bikes are what creates financial support for bike thieves, since all stolen bikes sold meet that description

All stolen bikes also have two wheels. I guess buying a bike with two wheels also creates a market for stolen bikes?

This is ridiculous. No one without papers can sell a bike according to you. I get that bike theft is a problem but you don't solve it by blaming innocent people. A lot of legitimate bikes are sold without papers.