This is a lesson I learned when I was 16. I worked as a "lot boy" for a marina. Cleaning up, moving stuff around, stuff like that. An old guy in grubby work cloths would come by almost daily with his lunch kit in hand and he would stare and stare at a boat that was worth then about $26K. This was the mid seventies, so that was a LOT of money. You could buy a house for that much in those days. Actually, you could buy a house and a car to put in the garage!
The sales people didn't bother with him, as he didn't look like he was capable of putting together $26 let alone $26,000! Finally, one day he asked me if he could buy the boat, so I took him into the sales manager's office. They talked for a while, and then the old guy opened up his lunch box and pulled out $25,000 in cash and said that was all he had and could he get a deal. Well, of course he could, and the deal was made.
Later the manager dragged me into the showroom where the sales guys were hanging around drinking coffee and paid me a 3% "commission" in cash using $10 and $20 bills (it took a while to count out) while he lectured them on not judging a book by it's cover. That was almost two month's pay for me in those days. So it left quite the impression on me. I don't know if the other sales people took it to heart, but they never let anyone on the lot without talking to them ever again. And they made darn sure nobody talked to me, either!