Don't have any definitive answer but I did try to find out something when I was considering sending a knife to the United States.
Finnish Postal Service website has some info on country-specific regulations and importing rules. Knives not a problem really, but I wouldn't try it with automatics and balisongs, those are the problematic ones everywhere.
Value has to be declared in the customs form. Small form for items under 300 Euros, larger and more detailed one if above 300 .
Letters: "Gifts" under 100 USD and "Personal Shipments" under 200 USD exempt from duty. Parcels: commercial shipments require sales invoice or
pro forma bill + other customs documents, tariff numbers etc. From what I understand, this would make it simpler for a couple of knifeknuts to make a personal deal, dealers have more concerns. But don't quote me on that!
No idea what the US import tariff on knives might be, at our end it's 8.5 %. Value Added Tax you don't have, I think?
Sending that way has the same problem with tracking and insurance that keeps many US members away from international deals. You can get tracking in some form if you send it as registered mail, but insurance is difficult. What I was told is that the USPS does not recognise insurance claims from over here. Probably due to a lack of a proper international agreement about those matters.
All in all, not impossible but it will require a bit more work.