Many warnings here re scams are apposite.

That being said the practice is very common, but almost always occurs either because the buyer & seller know and trust each other, or because the buyer knows somebody who has seen the sailplane and vouches for it.

The common problem from a buyer's POV is that if you are going to travel a long way to get a glider, you are in a very weak position to dicker when you get there. It's a whole lot smarter to have somebody you trust look it over for you, even if you pay them something ... than to drive a 1000 miles and then face going home empty handed, or upset that you wouldn't have paid that much "had you known."