My understanding is that Customs keep an eye on which models would normally be opened by gravity or "centrifugal force", so if it's just a matter of a tightened pivot screw they may see it is an attempt to mislead them.
Customs have recently made some changes to their policies in regard to how they're classifying things which should make it much more consistent. For a while it was the case that if it could literally be opened by one hand (including with a thumb stud/hole) it would be controlled. Now it's much more a matter of being able to flick the knife open. Spyderco Delica's and Dragonflies are coming through. I know of a few PM2's which have also made it.
I think Customs are now keeping a list of folding knives which are definitely controlled and those which would not normally be controlled. When I get a chance I will be writing to them to ask that they make that list public, if it isn't already.
Also, just to reiterate that fixed blades aren't normally a problem unless they're daggers (sharpened on two sides) or are of a kind which would be considered a fighting knife. Here are a couple links to the Australian Customs web page about weapons:
http://www.customs.gov.au/site/page4372.asp
http://www.customs.gov.au/site/page4372.asp#CommonWeaponsList
There are also a couple of broad reasons why a knife might be stopped by Customs:
1. It might be of a type controlled under the legislation governing Customs (eg in the case of one handed opening knives), or
2. It might be of a type considered a prohibited weapon under state legislation.
If the knife is stopped because of reason 2, then the buyer can often still get permission to get the item released to them if they show they have a legal reason to have the knife, depending on state legislation. For example here in South Oz I might be able to get some kinds of fighting knives through if I can show that I am a collector or have a reasonable need to use them in a recognized form of recreation. These circumstances are often very specific. There is also some jumping through hoops required. If you're the seller it is worth asking if a buyer has been refused by Customs outright or if they just have to go the extra mile to get law enforcement permission.
For what it's worth, as far as my own behaviour is concerned I don't care what Paypal rules are - if a knife is correctly described by the vendor and I buy it but it's seized and destroyed by Customs, I consider it my problem. I would find it personally unethical to expect the seller or knife maker to be out of pocket because of my mistake. If it was returned to the vendor I might negotiate towards a partial refund, but that would be on a case-by-case basis and that would be for the two of us to work out.
Australia is still a good place to do business. For what it's worth I've imported about 10 fixed blades from knife makers here and to date I haven't had any stopped.
(sorry about the marathon post)