seller etiquette

I have been reading here for a decade, first recent post under my newly found login.

I have contacted a seller about a subwoofer for a fair price. I asked if local pickup was an option. I got a response that it was. I wrote him that I would drive up, from 2 hours away, in 3 days. I told him I don't want to disturb him by plugging it in, I would just pay and pick it up, as I'd trust his word that it works. I was ok paying full price.

I then yesterday afternoon got an email that I needed to pay asap or he'd sell it to someone else as he got another offer. I didn't read the email until this morning when it had already been sold.

Is this normal? Fair? How could I have avoided it? Should have offered to pay in advance?  Should he had been waiting for my response and payment for a little longer?