Something I've started doing recently is video recording the boxing up process once I sell a knife. I'll video close ups of the knife to show it's condition and functionality, putting the knife back into it's packaging, wrapping that packaging with bubble wrap or paper if necessary, putting that into the shipping box, taping up the shipping box, putting the shipping label on the box (showing the addresses & tracking if it's a printed label), & weighing it once it's all boxed up. I do all of that in one take without ever letting the knife or anything get out of frame. I try to make it as definitive proof as I can that what I am shipping out is what I have described it to be to the buyer.
Only ship to a confirmed paypal address, and only ship once the payment has cleared. If you haven't sold much with paypal, they may put a hold on the funds until delivery is confirmed, but that's different than a "pending" payment I believe. Also, don't accept partial payments (down payments or payment installments), you will only have one tracking number and will only be able to apply it to one payment and a scammer could claim an item not received on all the other payments and paypal will refund them.
If you don't want to do international sales, you can configure your paypal setting to not accept payments from people with non US addresses and not accept payments in currencies that you do not hold (such as anything besides US dollars). You can also block payments from people with non US paypal accounts, however, some international buyers (with a non US paypal account) use a third party forwarding service that allows them to have a US shipping address for you to ship to. Again, I would only ship to that address if it is their confirmed paypal address. Be aware that your obligation for delivery ends once the package is delivered to that US address. Anything that gets lost or damaged from that point on is between them and their third party shipping service. If it arrives to their third party agent damaged, it needs to be reported back to you immediately so that you can file it as an insurance claim. Also be aware that if you do choose to do international sales, you should always claim full value of the item on the customs form and don't mark it as a gift. There's no need to put yourself at risk for lying on a customs form just so a buyer can save a few bucks on import taxes. You also need to use an expedited shipping service with reliable tracking, for international sales, in order to qualify for paypal protection. That can be very expensive, so be sure what that cost is before you agree on a final shipped price to the buyer.
Another good point that was brought to my attention recently is that you should never accept paypal friends & family no matter what excuse a buyer gives you. If a person wants you to ship to an unconfirmed address and claims that they will pay you via F&F, you will not be covered by paypal seller protection. The owner of that account could file an unauthorized transaction on that payment and say their account was hacked or used by an unauthorized person, which it very well could've been, and paypal will refund their money.
As far as selling to people that have little or no feedback, you really just have to trust your gut. Check some of their posting history to see if they are an actual contributing member of the forum. If they are, maybe you can get some insight as to what kind of person they are just by reading through some of their posts. Try googling their user id, email address, real name, or shipping address to see if you run across anything that they might've done anywhere else besides here. When you make a sales thread, don't list your email address. If you do, then even people that don't have an account here can contact you, which would leave you with one less way to check their background. Let them contact you by PM if they are able to, or comment in the sales thread with their contact information if they can't send pms.
I've bought and sold lots of knives here, as well as several other places online, and haven't been scammed on either side yet, but it can be very risky, so use every precaution you can to avoid getting tripped up by a scammer. Just remember, lots of scammers can be very persuasive and trick you into thinking they are trustworthy. Don't fall for it, you might have to turn down legitimate buyers sometimes in order to protect yourself from being scammed, but that's just how you have to be because that's what paypal dictates.
One more thing, if there is anything incorrect about a buyers address to where it won't let you print a shipping label, or isn't in the system (if you pay for postage at the post office), refund their money and have them fix their information and resubmit the payment. Don't change the zipcode, that's what paypal goes by to determine where you sent the package, if it's different than their confirmed address, they will refund the buyer. I don't know how strict paypal is on all this stuff, but that's pretty much how they have things laid out in their policies and I'm sure it would be more trouble than what it's worth to try to straighten anything out through customer service. Better safe than sorry.