If you pass a pickup truck on the highway carrying an overflowing load of bikes haphazardly strapped down you passed a professional bike thief. They are carrying a load of bikes from one town, often a college town, to another. If you've bought a bike from a Craigslist seller who turns out to have a dozen or more bikes locked to a rack, often on a college campus, you bought a bike stolen from a town down the road.
But these are very seldom expensive bikes. The bad news is that your $75 bike you think is too cheap to steal from the train station rack is exactly what this kind of bike thief is looking for. They want low risk and fast turnover.
While that is probably a factor, most of those places seem more like the guy who raids all the neighborhood trash days to pick up free bikes that will be thrown away, fixes them and sells.
But these are very seldom expensive bikes. The bad news is that your $75 bike you think is too cheap to steal from the train station rack is exactly what this kind of bike thief is looking for. They want low risk and fast turnover.