Might as well have them pay in advance, no? It's not a way to transfer a whole operational web application, in any case.

For relatively "small" amount of money, I totally agree. However, from my experience selling a tiny project [1], buyers were reluctant to transfer in advance even sums that are below $10K. Therefore we also used Escrow.com..

[1] I wrote more about my experience in my ebook - https://gum.co/side-project