Brian, in regards to negative feedback, most places try to discourage it. It is best to try and work with the seller first. Just remember to read their description of what they are selling. Some will tell you it's a small plant. It's up to you then to determine if you want to take the chance. In regards to a seller that doesn't want to budge on trying to diffuse the situation your only recourse is going to the auction "HONCHOS" to have them rectify the situation. If that don't work then a negative feedback is justified. WHO WANTS TO GO THRU ALL THAT !!!
I don't. Like you said block the seller and move on. There are many sellers out there that will bend over to make it right. I had a seller contacted me a couple years ago to let me know that they had "RUST" and that they would send me a refund. I told them to go ahead and send my plants anyway. The plants that I received didn't develop any "RUST" and I got a nice bonus plant to boot. That's being honest. Michele is also a perfect example.
In regards to the seller marking my bill as paid if they don't do it within a certain amount of time, like Michele said, I just do it myself. I put all my Paypal notification email in a save folder marked LA so that if there is any issue I can go thru there and sort it out. Despite all the negative, let's all have a great gardening season. Got my roses that I won on the raffle yesterday. They are looking great. Thanks Dave and Brushwood Gardens.