People will allow for a lot. Just let them know.
I understand when Hillary comes from, as I also have bad days and shitload stuff to do involving my family (but I don't try and make living from selling bike parts). I'm a bad buyer sometimes, but when I sell something, I try to stay on top of things - especially when people pay immediately after making dibs.
If they pay straight away, it means that they expect the same in return. Simples.
Payed asap before x-mas. Still no chainring.
Normally we buy things because we need them.