Essentially you are asking the seller to collaborate with you to commit tax/duty fraud.
You are also exposing the seller to the possibility of customs seizing the package for under declaring the value.
Your practice is wrong and possibly criminal.
The seller can always decline. If he doesn't, he gets the money he wants and I get my knife. Everybody happy. There have been no problems in the past so... Tax forms always say "pocket knife", which isn't a lie.
Furthermore, if the seller would mark the item as a "gift", I'd also evade customs so that would be fraud as well? But how can the people at customs know it wasn't really a gift but a purchase?
If the seller would undervalue, I doubt that -without opening the package- any customs officer would be able to see on an x-ray if the value of the knife is correct. Even if they would open the package, it would require another knife nut to declare if the item was undervalued...
I could also buy knives in the USA, let them be shipped to an adress in USA of some of the people I know in USA and make them send it as a gift to me.
Different methods, same result. Seller gets paid, I get the knife. Who is without sin, cast the first stone?
I could argue that politicians in my country often get into the spotlights with different corruption stories, fraud affaires etc. while the regular citizen is paying his last dime to taxes (which are very high over here), it would be true, but unhonest to hold this against the whole government. Pars pro totos can be made quite easily, government officials 'stealing' money from regular citizens, but they would be faulty arguments. My point is that a lot of stuff happens, I pay enough taxes already, politicians commit fraud quite regularly so I choose not to pay import taxes on my knives. My decision. Not 100% ethical? Sure.
Does it bother me or anyone else who I bought from online, not that I know.
So please, don't compare 'evading import customs in one-way-or-the-other' with real crimes such as murder, rape,... I'm not a criminal, just a student with a limited budget.