@parkergetdean
Admittedly, I did not read every single comment here, so some of this may have been covered already.
You asked "How could I avoid this in the future?" (Paraphrasing)
Here's what I would have done:
1) Offered to put down a small deposit (Venmo or PayPal) to guarantee holding until agree upon pickup date / time. Don't wait for the seller to ask for one - offer it yourself. This shows the seller that you are now vested in completing the transaction;
2) Did the seller know it was a 4 hour drive for you? Significant information like that should be communicated. If nothing else, it gives the seller understanding that with such a long distance to travel, he needs to be a little more flexible if (for example) you are an hour late getting to him due to traffic, etc.;
3) Unless the whole "I will just pick it up, won't even test it out" angle was crucial to getting the sellers agreement in the first place... which honestly would raise red flags in my mind IF IT WAS... why give up a generally recognized 'right' without getting anything in return? As a person who has both bought and sold in the past, I fully expect that anyone purchasing from me is going to want to test the equipment. If I was buying and a seller said "no testing, just pick it up and go..." that's a MAJOR downward push on pricing (at least what I would be willing to pay)... again, don't give up anything that is considered a standard allowance unless you are getting some consideration in return for giving it up...