If a seller wants to act like this is big business then they should adapt some of the things that other businesses are subject to, like the buyer's remorse law. Even with an automobile you get 3 days to change your mind. Now, to be clear, I've never changed my mind on a knife deal but I also don't think it makes anyone a pariah who should be forever shunned from purchasing from you again but, that's up to you. I've been around long enough to know that people change their minds about all kinds of things, some even MORE IMPORTANT than a knife purchase, believe it or not! ;-) Things like major purchases and even when it comes to MAJOR life decisions like marriage, etc. so, personally, I'm inclined to cut anyone slack who tells me they changed their mind. What do I know about their lives? Maybe they just got hit with a big, unexpected bill of some sort, the guy may have just been told his position at work has been eliminated, maybe the guy has an out of control knife buying habit, maybe he forgot to pay a bill that this knife purchase will delay causing him credit problems, I HAVE NO IDEA.
If this is a recurring theme with a buyer, THEN I'd certainly consider blocking them but not on one transaction. If I ran my business based on "one strike and you're out" I'm pretty sure that all those dentists who never heard of me before, would all of a sudden "know" something bad about me, were I to refuse to do business with one dentist due to some seemingly (to someone) petty issue. I give everyone the benefit of the doubt and don't run my business in such a way that should a client/customer NOT pay a bill, it won't negatively affect my bottom line in a drastic way. If the guy you blocked due to a change of heart on a knife sale, knows a lot of people, the negative word of mouth that a vindictive person could spread will hurt you more than that one lost sale. Besides, if you get a refund on the item AND shipping, what's the BIG problem? So you wasted a little time, I get it, it's annoying but who among this group has not wasted far more time combing this site and others, looking for that one knife that has elluded them? Some would say every minute you spend looking and not finding is wasted time.
As I said, I have a business unrelated to knives and so I'm sure my opinion will differ with many on here who are selling knives as a business. I buy knives I think I want to have and then, once I have it in hand, if it's one I've never handled before, I make the final decision as to whether or not it's a "keeper" which makes me a hobbyist. I only put knives up for sale that I decided that I no longer wanted to have. That said, even as a hobbyist, I am also a small business owner and generally use the same principles in my hobby sales as my business. I don't know about any of you but over the course of my almost 69 years here and going on 46 years of marriage, I've made decisions that I'd like to change but it was too late. That's the "school of hard knocks". In my hobbies, if at all possible, I'd like to keep it a little more casual than that. Don't get me wrong, if I get ripped off by someone, I won't sell to them again but a change of heart will not get me to that point but, again, that's just me and my opinion.
So, in my typically wordy fashion, I'm definetely inclined to give first offenders the benefit of the doubt and reevaluate from there. Who knows, with some understanding, you just might gain a long time customer and maybe even a friend. I will say that I've never been disappointed in my dealings with anyone on this site as a buyer. As a seller, I believe I've only ever sold one or two items and they too were without incident. Overall, I'd say we've got something good here, not perfect but, what is? I know I make mistakes and so assume others do as well and, as was stated in this thread at least once, I try to always 'do unto others as I would have them do unto me', to paraphrase a passage from a famous book.

Happy collecting and selling y'all, enjoy this cool hobby of ours.