First off, I make sure that the fish I am selling are healthy and of a good size. Have bought fish from club members before and they show up to the meeting with a bag of bug-sized fish. Completely ridiculous. Even if they do make the trip, that natural selection process has not had a chance to occur.
'Natural selection" meaning - in every batch of fry, no matter the care they receive, you have losses.... weak fish, deformed fish, runts, etc. By growing out, you sort of weed them out. Folks should not be selling African cichlids under 1" for this reason. In my opinion, 1.5" is pushing it.
When I'm selling a group of 1.5"-2" fish, do I ask to see their 75 gallon tank? No. Most people don't have their fish's final home available when they still have a year's worth of growth to do.
I'm more picky with adult fish that I sell. You put a lot of time into growing them out and want to see them go to a decent place.
Heck, after recent transactions, giving fish away or trading them to friends is more likely the path I'll be going. One of the reasons I haven't been pulling holding fish lately - easy to get a bad taste in your mouth.