Interesting. We buy both specific sets and pick up random bins at yard sales and whatnot. The sets we more or less try to keep together while we use the random bins for our own custom builds.
To combine several previous posts, at Christmas I’ll set out the Lego train set, which runs more reliably than the O or HO gauge sets I’ve had. Plus you can put Lego buildings and vehicles next to the track. I have the old fashioned powered track, but now they have battery operated locomotives with remote control.