After fourteen years on BF as a member, the only thing for certain is that you can't please all the people, all the time.
If you don't like the mutual agreement that you're going to enter on buying a knife and it doesn't sit well with you, don't buy it and move on. Otherwise, what two consenting adults agree to on a mutual basis about payment or delivery methods are between the buyer and seller. Pretty simple here.
This argument has run ad nauseam since paypal began and resurfaces every so often. End results? See the two sentences above.
Edited to add: Profit on knife sales? What is this word that you speak of?:highly_amused: Seriously though, if you're going to get bent over a couple of dollars either way on a knife deal stop collecting knives and take up accounting as a hobby. It will probably prove to be less stressful in the overall scheme of things.