seller etiquette

If you want traditional customer service - buy from a retailer or direct from the manufacturer. You want to save some bucks buying private, here is the only rule which matters: the seller doesn’t owe you anything until money changes hands. 

This one was probably mishandled on both sides. Frankly these future payment promises are annoying, and as seller I would not agree to it if I thought the item was desirable enough to get a faster sale. And if I were they buyer, I’d offer to pay up front if it really had to be a pickup in 3 days.

Don’t like it, don’t trust the seller enough? Pick a seller you do trust (feedback research), or go through retail channels. I know some guys don’t like to use shipping carriers but I generally prefer it - works well into the flow of a private transaction. Mishaps with UPS or FedEx are exceedingly rare with proper packing (factory packing). To be fair, a large subwoofer sucks to ship, especially if you’re going between east and west coasts ugh. 

as others have suggested, there's a different set of unwritten rules for established members, as opposed to new ones--i.e. if my prospective buyer had a long trail of positive feedback i would absolutely feel compelled to honor our deal, whereas if the buyer was an unknown, i'd probably have handle it like the op's seller. no aspersions on the op, who i'm sure is an honorable gentleman, but many of us have experienced phantom offerors who simply disappear.

I have done a few purchases that required a " drive " to acquire the item.

I usually reach an agreement where I will pay a deposit to secure the item and pay the balance upon pick-up..

Otherwise , you're just another tire kicker.

You have received good feedback from some experienced members, which I believe was your objective.

• A private seller is not the same as a retailer or the manufacturer. 
• At least the seller tried to contact you before selling it and to let you know not to make the drive, which is more than some would do. 
• Clear communication is important but money speaks loudest.
• If you want to lock in your sale, a down payment is good and full payment is best.

These may not be the answers you were looking for but many of us have been successfully doing this for many years.  Along the way, we have all learned lessons the hard way.  Good luck on your future endeavors.

Unfortunately, money talks.  There is no way that I would pay the full price and then pick it up at a later date.  That's just asking for trouble.  Perhaps you could have venmoed a small deposit to show your sincerity.